feat: add admin provider setup UI with improved product sync

- Add /admin/providers LiveView for connecting and managing POD providers
- Implement pagination for Printify API (handles all products, not just first page)
- Add parallel processing (5 concurrent) for faster product sync
- Add slug-based fallback matching when provider_product_id changes
- Add error recovery with try/rescue to prevent stuck sync status
- Add checksum-based change detection to skip unchanged products
- Add upsert tests covering race conditions and slug matching
- Add Printify provider tests
- Document Printify integration research (product identity, order risks,
  open source vs managed hosting implications)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

PROGRESS.md

@@ -12,16 +12,14 @@
 - 100% PageSpeed score
 
 **In Progress:**
-- Products context with provider integration (Phase 1 complete)
+- Products context with provider integration (sync working, wiring to shop views next)
 
 ## Next Up
 
-1. **Admin Provider Setup UI** (`/admin/providers`) - Add/edit/test provider connections
-2. **Product Sync Strategy:**
-   - ProductSyncWorker (Oban) for manual/scheduled sync
-   - Webhook endpoint for real-time updates from Printify
-3. Wire Products context to shop LiveViews (replace PreviewData)
-4. Session-based cart with real variants
+1. **Printify Webhook Endpoint** - Real-time product updates when changes happen in Printify
+2. **Wire Products to Shop LiveViews** - Replace PreviewData with real synced products
+3. **Variant Selector Component** - Size/colour picker on product pages
+4. **Session-based Cart** - Real cart with actual variants
 
 ---
 
@@ -57,15 +55,25 @@ See: [docs/plans/image-optimization.md](docs/plans/image-optimization.md) for im
 - [x] Provider abstraction layer
 - [x] Printify client integration
 - [x] Product/variant/image schemas
+- [x] Admin Provider Setup UI (`/admin/providers`) - connect, test, sync
+- [x] ProductSyncWorker with pagination, parallel processing, error recovery
+- [x] Slug-based fallback matching for changed provider IDs
 
 #### Remaining Tasks
-- [ ] **Admin Provider Setup UI** - `/admin/providers` for managing connections (~2hr) ← **NEXT**
-- [ ] Create ProductSyncWorker (Oban) for manual/scheduled sync (~1hr)
 - [ ] Printify webhook endpoint for real-time product updates (~1.5hr)
 - [ ] Wire shop LiveViews to Products context (~2hr)
 - [ ] Add variant selector component (~2hr)
 
+#### Future Enhancements (post-MVP)
+- [ ] Pre-checkout variant validation (verify availability before order)
+- [ ] Cost change monitoring/alerts (warn if Printify cost increased)
+- [ ] OAuth platform integration (appear in Printify's "Publish to" UI)
+
+#### Technical Debt
+- [ ] Add HTTP mocking (Mox/Bypass) for Printify API tests
+
 See: [docs/plans/products-context.md](docs/plans/products-context.md) for implementation details
+See: [docs/plans/printify-integration-research.md](docs/plans/printify-integration-research.md) for API research & risk analysis
 
 ### Cart & Checkout
 **Status:** Planned