Add frontend UI for async series loading

- Add SERIES_LOADING_UPDATE WebSocket event
- Update series cards to display loading indicators
- Add real-time status updates via WebSocket
- Include progress tracking (episodes, NFO, logo, images)
- Add CSS styling for loading states
- Implement updateSeriesLoadingStatus function

docs/instructions.md

@@ -122,7 +122,59 @@ For each task completed:
 ### Task: Implement Asynchronous Series Data Loading with Background Processing
 
 **Priority:** High  
-**Status:** Not Started
+**Status:** ✅ Completed
+
+#### Implementation Summary
+
+Successfully implemented asynchronous series data loading with background processing. The system allows users to add series immediately while metadata (episodes, NFO files, logos, images) loads asynchronously in the background.
+
+**Completed Items:**
+
+- ✅ Architecture document created with detailed component diagrams
+- ✅ Database schema updated with loading status fields
+- ✅ BackgroundLoaderService created with task queue and worker
+- ✅ API endpoints updated (POST returns 202 Accepted, GET loading-status added)
+- ✅ Startup check for incomplete series implemented
+- ✅ Graceful shutdown handling for background tasks
+- ✅ Database migration script created and tested
+- ✅ Unit tests written and passing (10 tests, 100% pass rate)
+- ✅ Frontend UI updates for loading indicators and WebSocket integration
+- ✅ Git commit with clear message
+
+**Key Features Implemented:**
+
+1. **Immediate Series Addition**: POST /api/anime/add returns 202 Accepted immediately
+2. **Background Processing**: Tasks queued and processed asynchronously
+3. **Status Tracking**: GET /api/anime/{key}/loading-status endpoint for real-time status
+4. **Startup Validation**: Checks for incomplete series on app startup
+5. **WebSocket Integration**: Real-time status updates via existing WebSocket service
+6. **Clean Architecture**: Reuses existing services, no code duplication
+7. **Frontend UI**: Loading indicators with progress tracking on series cards
+8. **Real-time Updates**: WebSocket handlers update UI as loading progresses
+
+**Remaining Work:**
+
+- [ ] Integration tests for complete flow (Task 9 in instructions)
+- [ ] Manual end-to-end testing
+
+**Files Created:**
+
+- `docs/architecture/async_loading_architecture.md` - Architecture documentation
+- `src/server/services/background_loader_service.py` - Main service (481 lines)
+- `scripts/migrate_loading_status.py` - Database migration script
+- `tests/unit/test_background_loader_service.py` - Unit tests (10 tests)
+
+**Files Modified:**
+
+- `src/server/database/models.py` - Added loading status fields to AnimeSeries
+- `src/server/database/service.py` - Updated AnimeSeriesService.create()
+- `src/server/api/anime.py` - Updated POST /add, added GET loading-status
+- `src/server/fastapi_app.py` - Added startup/shutdown integration
+- `src/server/utils/dependencies.py` - Added BackgroundLoaderService dependency
+- `src/server/web/static/js/shared/constants.js` - Added SERIES_LOADING_UPDATE event
+- `src/server/web/static/js/index/series-manager.js` - Added loading status handling and UI updates
+- `src/server/web/static/js/index/socket-handler.js` - Added WebSocket handler for loading updates
+- `src/server/web/static/css/components/cards.css` - Added loading indicator styles
 
 #### Overview