Milestone 3: catalog sync, batch transactions, and reports

- GET /api/v1/catalog/sync?since= — delta sync for Android offline-first
- POST /api/v1/transactions/batch — idempotency-keyed batch upload (207 Multi-Status),
  validates product ownership, skips duplicates silently
- GET /api/v1/transactions + /reports/summary — paginated list and aggregated
  revenue/tax/top-product reporting with date range filters
- ReportsPage: stat cards, payment method breakdown, top-10 products, transaction table
- Reports added to sidebar nav and router

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ROADMAP.md

@@ -25,13 +25,15 @@
 
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-## Milestone 3 — Android & Offline Sync
-- [ ] `GET /api/v1/catalog/sync?since=` for delta syncs
-- [ ] `POST /api/v1/transactions/batch` with idempotency keys
-- [ ] Android Kotlin app: MVVM, Room, offline-first flows
+## Milestone 3 — Android & Offline Sync ✅ (server-side)
+- [x] `GET /api/v1/catalog/sync?since=<ISO>` — delta sync for products, categories, taxes
+- [x] `POST /api/v1/transactions/batch` — idempotency-keyed batch upload (207 Multi-Status)
+- [x] `GET /api/v1/transactions` — paginated list with date/status/payment filters
+- [x] `GET /api/v1/transactions/reports/summary` — revenue, tax, top products, payment breakdown
+- [x] Reports page: stat cards, payment method breakdown, top products, transaction table
+- [ ] Android Kotlin app: MVVM, Room, offline-first flows (separate deliverable)
 - [ ] Background sync worker (Android)
-- [ ] Conflict resolution strategy: server-authoritative for payments
-- [ ] Admin reporting: Android-originated transactions appear in reports
+- [ ] Conflict resolution: server-authoritative for payments (enforced via idempotency)
 
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