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+# Architecture Reference
+
+_Last updated: 2026-09-23 (v1.5.9)_
+
+## Data Flow
+
+```
+Disco DB (open jobs)
+        │
+        ├─► ServiceTrackerService.BuildDashboard()
+        │         │
+Google Sheet CSV   │         ├─► BuildDiscoTile()   ─┐
+        │         │         └─► BuildSheetTile()  ─┤
+        ▼         │                                  ▼
+  GoogleSheetService                         List<DashboardTile>
+  (fetch → cache → parse)                           │
+                                                     ▼
+                                           Bucket sort into:
+                                           - ReadyForReturn
+                                           - OnHold
+                                           - AwaitingVendor
+                                           - AwaitingParts
+                                           - Tiles (main grid)
+                                                     │
+                                                     ▼
+                                          DashboardViewModel
+                                                     │
+                                                     ▼
+                                         WebHandler → HTML string
+```
+
+## Persistence (JSON files)
+
+All under `App_Data/Plugins/Disco.Plugins.ServiceTracker/`:
+
+| File | Contents |
+|------|---------|
+| `config.json` | `ServiceTrackerConfig` — priorities, locations, statuses, technicians, sheet config |
+| `tickets/` | One JSON file per Disco job (`{jobId}.json`) — `ServiceTicket` overrides, notes, changelog |
+| `external_tickets/` | One JSON file per NTT sheet row (`{internalId}.json`) — same shape as ServiceTicket |
+| `sheet_cache.csv` | Last fetched CSV from Google; invalidated by `RefreshMinutes` |
+
+## ID Generation for NTT Tickets
+
+Stable integer ID from djb2 hash of `timestamp|email|task|issue` (lowercased, trimmed), modulo 800000 + 100000 offset. Collisions resolved by incrementing. IDs are stable across refreshes as long as those four fields don't change.
+
+## Column Index Contract
+
+All Google Sheet column indices live in `GoogleSheetConfig`. Current mapping (updated 2026-09-23):
+
+```
+0  ColTimestamp      — Timestamp
+1  ColEmail          — Email address
+2  ColTask           — What is the task/request?
+3  ColIssue          — Detailed Description
+4  ColLocation       — Location / Classroom
+5  ColRequestedBy    — Requested By
+6  ColPriority       — Priority Level
+7  ColPreferredDate  — Preferred Completion Date
+8  ColCampus         — Campus (location fallback if ColLocation empty)
+9  ColAssignedTo     — Assigned to
+10 ColStatus         — Status
+11 —                 — Column 1 (junk, ignored)
+12 —                 — Campus duplicate (ignored)
+13 ColNotes          — Notes
+14 —                 — Category (ignored)
+15 —                 — Resolution Confirmed (ignored)
+ColCompletionDate = -1  (no dedicated column; disabled)
+```
+
+Set any index to `-1` to disable it. `SafeGet()` returns null for index < 0.
+
+## Sidebar Bucket Logic
+
+Buckets are evaluated in priority order and are mutually exclusive:
+
+```csharp
+rfr           = status contains "ready for return" OR "ready for pickup"
+onHold        = status contains "on hold"            AND NOT in rfr
+awaitingVendor= status contains "awaiting vendor"    AND NOT in rfr, onHold
+awaitingParts = status contains "awaiting parts"     AND NOT in rfr, onHold, awaitingVendor
+main          = everything else
+```
+
+Status is checked against `StatusOverride ?? DiscoStatus`.
+
+## NTT Closed Ticket Suppression
+
+NTT tickets are skipped in `BuildDashboard` if their saved `StatusOverride` contains any of:
+`resolved, dismissed, complete, closed, done, cancel, finished`
+
+This is independent of the sheet's own status column — it prevents jobs dismissed in the tracker from reappearing on every sheet refresh.
+
+## Location Resolution for NTT Tiles
+
+```
+ExternalTicket.Location (from ColLocation)
+  └─ empty? → try ColCampus
+       └─ still empty? → null
+
+MatchSheetLocation(raw):
+  1. Exact ID match against cfg.Locations
+  2. Exact Name match (case-insensitive)
+  3. Substring match either direction
+
+  Match found  → use matched location (id/name/icon/colour)
+  No match     → use DefaultLocation + store raw in RawSheetLocation
+                 (displayed as italic 📍 pill on tile)
+
+Saved LocationId override (from external_tickets/) always wins over sheet text.
+```
+
+## HTML Generation Pattern
+
+No Razor. Every page is a method returning `string` from a `StringBuilder`:
+- `BuildDashboardPage(DashboardViewModel)` — main dashboard
+- `BuildDetailPage(Job, ServiceTicket, ...)` — job detail + edit
+- `BuildConfigPage(ServiceTrackerConfig, ...)` — settings
+- `TileHtml(DashboardTile)` — single tile card
+- `SidebarPanel(title, color, items, url, showCollected)` — reusable sidebar section
+
+CSS is returned from `CSS()`, `DCSS()`, `CCSS()` string methods and inlined in `<style>` tags. No external stylesheets.
+
+## Cache Invalidation
+
+`DashboardCache` watches file modification times on `config.json`, `external_tickets.json`, and the tickets directory. Any file change invalidates the cache. Cache key includes sortBy + filter string.
+
+## Priority SLA Logic
+
+Each `PriorityLevel` has a `SlaHours` value. SLA deadline = `job.OpenedDate + SlaHours`. Warning fires at 75% elapsed. `SlaHours = 0` means no SLA (used for "Scheduled" priority).