diff --git a/app/services/page_order.py b/app/services/page_order.py
new file mode 100644
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+++ b/app/services/page_order.py
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+"""
+Page ordering for image-based instructions.
+
+Images arrive in whatever order the browser handed them over, so page_number
+starts out as "order of arrival" rather than "order of the manual". Everything
+that fixes that lives here: the sort strategies, the renumbering, and the
+single apply_order() that every route and CLI command funnels through.
+
+One rule throughout: page numbers for a set are always a dense 1..n sequence
+with no gaps, no duplicates and no NULLs. Any function that touches the order
+leaves the set in that state.
+
+Only file_type == 'IMAGE' rows are ordered. PDFs are whole manuals, not pages.
+
+Created by: Jess Rogerson (yelling commands at Claude.AI)
+"""
+
+import os
+import re
+from typing import Dict, Iterable, List, Optional, Sequence
+
+from app import db
+from app.models.instruction import Instruction
+
+
+# --- filename reading -------------------------------------------------------
+
+# A number that was clearly meant as a page marker: "page 7", "pg_07", "p-7",
+# "7 of 48", "7/48". Checked before falling back to "last number in the name".
+_PAGE_MARKERS = (
+    re.compile(r'(?:^|[^a-z0-9])(?:page|pg|p)[\s._\-]*(\d{1,4})(?![0-9])', re.I),
+    re.compile(r'(?:^|[^a-z0-9])(\d{1,4})\s*(?:of|_of_|/)\s*\d{1,4}', re.I),
+)
+
+_DIGITS = re.compile(r'(\d+)')
+
+
+def natural_key(name: str) -> tuple:
+    """
+    Sort key that reads runs of digits as numbers, so page2 lands before
+    page10 instead of after it.
+
+    Each token becomes a 3-tuple so numbers and text never get compared
+    against each other, which would raise TypeError on mixed names.
+    """
+    key = []
+    for token in _DIGITS.split(name or ''):
+        if token.isdigit():
+            key.append((0, int(token), ''))
+        elif token:
+            key.append((1, 0, token.lower()))
+    return tuple(key)
+
+
+def page_hint(name: str) -> Optional[int]:
+    """
+    The page number a filename is claiming to be, or None.
+
+    Tries explicit markers first ("page_12"), then falls back to the last
+    number in the name, which covers camera and scanner names like
+    IMG_2043.jpg or scan-0012.png. The value may not be a real page number,
+    but for a batch of files from one source it still sorts correctly.
+    """
+    stem = os.path.splitext(name or '')[0]
+
+    for pattern in _PAGE_MARKERS:
+        match = pattern.search(stem)
+        if match:
+            return int(match.group(1))
+
+    numbers = _DIGITS.findall(stem)
+    return int(numbers[-1]) if numbers else None
+
+
+def _display_name(instruction: Instruction) -> str:
+    """The original upload name, falling back to the stored filename."""
+    if instruction.file_name:
+        return instruction.file_name
+    return os.path.basename((instruction.file_path or '').replace('\\', '/'))
+
+
+# --- sort strategies --------------------------------------------------------
+#
+# Every key ends with instruction.id so the result is stable and repeatable
+# when two files tie.
+
+def _key_filename(instruction: Instruction) -> tuple:
+    return (natural_key(_display_name(instruction)), instruction.id)
+
+
+def _key_page_hint(instruction: Instruction) -> tuple:
+    hint = page_hint(_display_name(instruction))
+    # Files with no number at all go to the back rather than the front.
+    return (hint is None, hint or 0,
+            natural_key(_display_name(instruction)), instruction.id)
+
+
+def _key_uploaded(instruction: Instruction) -> tuple:
+    return (instruction.uploaded_at is None, instruction.uploaded_at,
+            instruction.id)
+
+
+def _key_size(instruction: Instruction) -> tuple:
+    return (instruction.file_size or 0, instruction.id)
+
+
+STRATEGIES = {
+    'filename': ('Filename (natural order)', _key_filename),
+    'page_hint': ('Page number found in filename', _key_page_hint),
+    'uploaded': ('Upload order (oldest first)', _key_uploaded),
+    'size': ('File size (smallest first)', _key_size),
+}
+
+# Applied after sorting rather than as a key, so it can also reverse the
+# order currently on screen.
+REVERSIBLE = 'reverse'
+
+
+def strategy_choices() -> List[tuple]:
+    """(value, label) pairs for the sort menu."""
+    return [(value, label) for value, (label, _) in STRATEGIES.items()]
+
+
+# --- reading the current order ----------------------------------------------
+
+def _current_key(instruction: Instruction) -> tuple:
+    """Current order: by page number, NULLs last, id as tie-break."""
+    return (instruction.page_number is None, instruction.page_number or 0,
+            instruction.id)
+
+
+def image_pages(set_id: int) -> List[Instruction]:
+    """Every image instruction for a set, in its current page order."""
+    rows = Instruction.query.filter_by(set_id=set_id, file_type='IMAGE').all()
+    return sorted(rows, key=_current_key)
+
+
+def audit(set_id: int) -> Dict:
+    """
+    What's wrong with the current numbering, for the UI banner and the CLI.
+
+    Returns total, plus the duplicate numbers, the gaps in the sequence, and
+    how many pages have no number at all. is_clean is True when the set is a
+    dense 1..n run.
+    """
+    pages = image_pages(set_id)
+    numbers = [p.page_number for p in pages if p.page_number is not None]
+
+    seen, duplicates = set(), set()
+    for number in numbers:
+        if number in seen:
+            duplicates.add(number)
+        seen.add(number)
+
+    gaps = []
+    if numbers:
+        gaps = [n for n in range(1, max(numbers) + 1) if n not in seen]
+
+    unnumbered = sum(1 for p in pages if p.page_number is None)
+
+    return {
+        'total': len(pages),
+        'duplicates': sorted(duplicates),
+        'gaps': gaps,
+        'unnumbered': unnumbered,
+        'is_clean': not duplicates and not gaps and not unnumbered,
+    }
+
+
+# --- writing a new order ----------------------------------------------------
+
+def apply_order(set_id: int, ordered_ids: Sequence, commit: bool = True) -> List[Instruction]:
+    """
+    Write ordered_ids to page_number as 1..n.
+
+    ordered_ids may be partial: any image belonging to the set that isn't
+    listed keeps its relative order and is appended after the listed ones.
+    That way a half-finished drag never strands a page at number 0.
+
+    Raises ValueError on an id that isn't an image page of this set, or on a
+    duplicate id.
+    """
+    pages = image_pages(set_id)
+    by_id = {page.id: page for page in pages}
+
+    wanted: List[Instruction] = []
+    taken = set()
+
+    for raw_id in ordered_ids or []:
+        try:
+            page_id = int(raw_id)
+        except (TypeError, ValueError):
+            raise ValueError(f'Not a page id: {raw_id!r}')
+
+        if page_id not in by_id:
+            raise ValueError(f'Page {page_id} is not an image page of this set')
+        if page_id in taken:
+            raise ValueError(f'Page {page_id} listed more than once')
+
+        taken.add(page_id)
+        wanted.append(by_id[page_id])
+
+    # Anything the caller left out keeps its current relative position.
+    wanted.extend(page for page in pages if page.id not in taken)
+
+    for position, page in enumerate(wanted, start=1):
+        if page.page_number != position:
+            page.page_number = position
+
+    if commit:
+        db.session.commit()
+
+    return wanted
+
+
+def renumber(set_id: int, commit: bool = True) -> List[Instruction]:
+    """
+    Close gaps and clear duplicates without changing the visible order.
+
+    This is the repair job: the pages stay in the order they're already in,
+    they just end up numbered 1..n.
+    """
+    return apply_order(set_id, [p.id for p in image_pages(set_id)], commit=commit)
+
+
+def auto_sort(set_id: int, strategy: str = 'filename',
+              reverse: bool = False, commit: bool = True) -> List[Instruction]:
+    """
+    Reorder a set's pages by one of the STRATEGIES, then renumber 1..n.
+
+    Raises ValueError on an unknown strategy.
+    """
+    if strategy not in STRATEGIES:
+        raise ValueError(f'Unknown sort strategy: {strategy}')
+
+    _, key = STRATEGIES[strategy]
+    pages = sorted(image_pages(set_id), key=key, reverse=reverse)
+    return apply_order(set_id, [p.id for p in pages], commit=commit)
+
+
+def reverse_order(set_id: int, commit: bool = True) -> List[Instruction]:
+    """Flip the current order end to end. Fixes back-to-front scan batches."""
+    pages = list(reversed(image_pages(set_id)))
+    return apply_order(set_id, [p.id for p in pages], commit=commit)
+
+
+def move_page(instruction_id: int, position: int, commit: bool = True) -> List[Instruction]:
+    """
+    Put one page at a given 1-based position and shuffle the rest around it.
+
+    position is clamped to the ends, so 0 or 999 both do something sensible.
+    Raises ValueError if the instruction isn't an image page.
+    """
+    page = Instruction.query.get(instruction_id)
+    if page is None or page.file_type != 'IMAGE':
+        raise ValueError('That instruction is not an image page')
+
+    pages = image_pages(page.set_id)
+    remaining = [p for p in pages if p.id != page.id]
+
+    index = max(0, min(int(position) - 1, len(remaining)))
+    remaining.insert(index, page)
+
+    return apply_order(page.set_id, [p.id for p in remaining], commit=commit)
+
+
+def swap_pages(set_id: int, first_id: int, second_id: int,
+               commit: bool = True) -> List[Instruction]:
+    """Exchange two pages, leaving everything else alone."""
+    pages = image_pages(set_id)
+    ids = [p.id for p in pages]
+
+    try:
+        first_index = ids.index(int(first_id))
+        second_index = ids.index(int(second_id))
+    except ValueError:
+        raise ValueError('Both pages must belong to this set')
+
+    ids[first_index], ids[second_index] = ids[second_index], ids[first_index]
+    return apply_order(set_id, ids, commit=commit)
+
+
+def next_page_number(set_id: int) -> int:
+    """
+    The number a newly uploaded image should get.
+
+    Uses the count rather than max() + 1 so a set that was previously left
+    with gaps doesn't keep growing them.
+    """
+    return len(image_pages(set_id)) + 1