Phases 2-4: Requester support, batches, webhooks, scraper registry

Phase 2 - Requester & Batch Support:
- core/database.py: Added create_job params (requester_*, batch_*, priority, callback_*)
- core/database.py: Added batch methods (create_batch, get_batch, update_batch_progress, get_batches)
- core/database.py: Added update_job_callback for tracking webhook delivery
- api/routes/batches.py: New endpoints:
  - POST /api/scrape/google-reviews/batch (submit batch)
  - GET /api/batches (list batches)
  - GET /api/batches/{id} (batch detail)
  - DELETE /api/batches/{id} (cancel batch)
- api_server_production.py: Updated /api/scrape with requester, priority, callback fields
- api_server_production.py: New primary endpoint POST /api/scrape/google-reviews

Phase 3 - Webhooks:
- services/job_callback_service.py: New service with:
  - JobCallbackService: send_job_callback, send_batch_callback, retry_failed_callbacks
  - JobCallbackDispatcher: Background worker for callback monitoring
  - Payload formats per spec (job.completed, job.failed, batch.completed)
  - Exponential backoff for retries
  - Error classification for failure payloads

Phase 4 - Scraper Registry:
- scrapers/registry.py: Database-backed version routing:
  - get_scraper(): Version/variant/A/B routing
  - _get_weighted_scraper(): Traffic-weighted random selection
  - 60-second TTL cache for performance
  - register_scraper, deprecate_scraper, update_traffic_allocation
  - LegacyScraperRegistry preserved for backwards compatibility

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

.artifacts/CONTEXT-KEEPER.md

@@ -104,7 +104,7 @@ reviewiq/                          # Will rename from google-reviews-scraper-pro
 | Phase | Description | Status |
 |-------|-------------|--------|
 | 0 | Project restructure (move files to new locations) | ✅ COMPLETE |
-| 1 | Database migrations (new fields + tables) | Not started |
+| 1 | Database migrations (new fields + tables) | ✅ COMPLETE |
 | 2 | Requester & batch support | Not started |
 | 3 | Webhooks | Not started |
 | 4 | Scraper versioning & registry | Not started |