Document the parts labelling feature

docs/PARTS_LABELS.md

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+# Parts Drawer Labels
+
+Admin-only labelling system for sorting drawers and bags. Browse a part
+taxonomy, define a label per drawer, and print them onto sticker sheets.
+
+Lives at **`/admin/labels`** — in the admin menu as *Parts Labels*.
+
+---
+
+## Deploying
+
+The feature is already wired into `app/__init__.py`, `schema_upgrade.py` and
+`base.html`, so a normal pull is enough:
+
+```
+git pull
+```
+
+Restart LIM. The tables are created automatically at boot by
+`app/services/schema_upgrade.py`, and the taxonomy seeds itself the first time
+an admin opens the page.
+
+If you'd rather make the database change deliberately before restarting:
+
+```
+python migrate_add_part_labels.py
+```
+
+Optional extras:
+
+| What | Why | How |
+|------|-----|-----|
+| Rebrickable API key | Part lookup: numbers, names, artwork | `REBRICKABLE_API_KEY=...` in `.env`, or paste it into *Labels → Settings* |
+| ReportLab | PDF export button | `pip install reportlab` |
+
+Neither is required. Without them you type labels in by hand, upload your own
+images, and print from the browser.
+
+---
+
+## Why not Brickset?
+
+Brickset's API v3 covers sets, minifigs and instructions. It has **no parts
+endpoint**, so it can't supply part numbers, names or part images.
+[Rebrickable](https://rebrickable.com/api/) can, and a key is free. Brickset
+stays where it is, doing sets.
+
+## Why not Brick Architect's labels?
+
+Tom Alphin's label PDFs and his category numbering are his own copyrighted
+work. This feature doesn't scrape, copy or redistribute them. What ships is a
+starter taxonomy built from standard LEGO/BrickLink part terminology, arranged
+in the same *style* of hierarchy, and completely editable — rename, renumber,
+recolour and restructure it from *Labels → Categories* so it matches how your
+drawers actually run. If you want his exact labels, download them from his site
+and use them; this is for generating your own.
+
+---
+
+## Day-to-day
+
+**Adding labels**
+
+- *New Label* — name, dimensions, part number, alternates, category, drawer,
+  colour, image.
+- Or, with a Rebrickable key, search on the labels page and hit *Add label* on
+  a result. It pulls the name, number and image, caches the image locally so
+  printing works offline, then drops you on the edit form to set the drawer.
+
+**The `name` + `dimensions` split matters.** Name is the big bold line
+("Brick", "Plate, Modified with Clip"); dimensions is the geometry ("2 x 4").
+They're printed together but stored apart so you can turn dimensions off for
+mixed drawers.
+
+**Alternate part numbers** are for old moulds that live in the same drawer —
+`3001a, 3001b`. Comma separated, searchable, optionally printed.
+
+**Printing**
+
+1. Tick labels (or *Print whole category*).
+2. Pick a sheet, set the options, hit *Update preview*.
+3. Print from the browser: **margins None, scale 100%** — not "Fit to page",
+   which is what ruins most label sheets.
+
+Options worth knowing:
+
+- **Skip positions** — start partway down a part-used sheet instead of wasting it.
+- **Copies each** — same label on several drawers.
+- **Offset X/Y** — per-printer alignment nudge in millimetres. Set your usual
+  value once in *Settings*.
+- **Outline each label** — hairlines for cutting, and for test-printing on plain
+  paper before you commit a sheet of Averys.
+- **Show on each label** — image, name, dimensions, part number, alternates,
+  category, category code, drawer, colour, category colour stripe.
+
+**Sheet presets**: Avery L7651 (65/sheet, good for bags), L7160 (21/sheet, the
+all-rounder), L7163 (14/sheet, wide), L7159 (24/sheet), L7169 (8/sheet, large),
+Avery 5160 (US Letter), plus two plain-A4 grids for cut-your-own sticker paper
+or paper-and-tape.
+
+---
+
+## Where things live
+
+```
+app/models/part_label.py            PartCategory (tree) + PartLabel
+app/services/part_catalog.py        taxonomy seeding, Rebrickable client
+app/services/label_render.py        sheet geometry, layout, optional PDF
+app/routes/labels.py                the blueprint (all admin_required)
+app/data/part_taxonomy.json         the starter taxonomy
+app/templates/admin/labels/         index, form, categories, settings, print
+<UPLOAD_FOLDER>/parts/              cached and uploaded part images
+```
+
+Labels are **not** an inventory. There's no quantity field on purpose — the
+drawer tells you how many you have.
+
+---
+
+Created by: Jess Rogerson (yelling commands at Claude.AI)