feat: add notifications query with rkey-based sorting
Add cross-collection notifications query that finds records mentioning a DID:
- notifications(viewerDid, collections, first, after) GraphQL query
- NotificationRecord union type for type-safe results
- RecordCollection enum for collection filtering
- Real-time notificationCreated subscription with event filtering
Sort notifications by rkey (TID) for chronological ordering:
- Add rkey generated column to record table (VIRTUAL for SQLite, STORED for PostgreSQL)
- Update pagination to support rkey field extraction
- Cursor pagination uses rkey|uri format
Includes comprehensive test coverage for repository, e2e, and subscription handlers.