feat: Complete watchlist & auto-download system with UI

Implement comprehensive watchlist system with automatic episode detection
and downloading. Features include per-user watchlists, scheduler-based
periodic checks, and a modern web UI.

**Backend Components:**
- WatchlistManager: JSON-based storage with multi-tenant support
- EpisodeChecker: Detects and downloads new episodes automatically
- AutoDownloadScheduler: APScheduler-based periodic task execution
- Complete REST API for CRUD operations and scheduler control

**Frontend Components:**
- Modern watchlist page with dark theme and animations
- Real-time status updates and progress tracking
- Scheduler controls with next-run display
- Add anime directly from search results

**Models & Configuration:**
- WatchlistItem with status, quality, and auto-download settings
- WatchlistSettings for global configuration
- Per-user statistics and provider tracking

**API Endpoints:**
- GET/POST /api/watchlist - List and add items
- PUT/DELETE /api/watchlist/{id} - Update and delete
- POST /api/watchlist/{id}/check - Manual check trigger
- POST /api/watchlist/check-all - Check all due items
- GET/PUT /api/watchlist/settings - Global settings
- GET /api/watchlist/stats - Statistics
- GET/POST /api/watchlist/scheduler/* - Scheduler control

**Configuration Files:**
- config/watchlist.json - User watchlist data
- config/watchlist_settings.json - Global settings

Generated with [Claude Code](https://claude.ai/code)
Co-Authored-By: Claude <noreply@anthropic.com>

app/watchlist.py

@@ -54,15 +54,16 @@ class WatchlistManager:
             self.watchlist = {}
 
     def _save_watchlist(self):
-        """Save watchlist to JSON file"""
         try:
             os.makedirs(os.path.dirname(self.db_file), exist_ok=True)
             data = {
                 item_id: item.model_dump(mode='json')
                 for item_id, item in self.watchlist.items()
             }
-            with open(self.db_file, 'w', encoding='utf-8') as f:
+            temp_file = f"{self.db_file}.tmp"
+            with open(temp_file, 'w', encoding='utf-8') as f:
                 json.dump(data, f, indent=2, ensure_ascii=False, default=str)
+            os.replace(temp_file, self.db_file)
             logger.debug(f"Saved {len(self.watchlist)} items to watchlist")
         except Exception as e:
             logger.error(f"Error saving watchlist: {e}")
@@ -84,11 +85,12 @@ class WatchlistManager:
             self.settings = WatchlistSettings()
 
     def _save_settings(self):
-        """Save watchlist settings to JSON file"""
         try:
             os.makedirs(os.path.dirname(self.settings_file), exist_ok=True)
-            with open(self.settings_file, 'w', encoding='utf-8') as f:
+            temp_file = f"{self.settings_file}.tmp"
+            with open(temp_file, 'w', encoding='utf-8') as f:
                 json.dump(self.settings.model_dump(mode='json'), f, indent=2, ensure_ascii=False)
+            os.replace(temp_file, self.settings_file)
             logger.debug("Saved watchlist settings")
         except Exception as e:
             logger.error(f"Error saving settings: {e}")