Add MP4 scan progress visibility in UI

- Add broadcast_scan_started, broadcast_scan_progress, broadcast_scan_completed to WebSocketService
- Inject WebSocketService into AnimeService for real-time scan progress broadcasts
- Add CSS styles for scan progress overlay with spinner, stats, and completion state
- Update app.js to handle scan events and display progress overlay
- Add unit tests for new WebSocket broadcast methods
- All 1022 tests passing

src/server/services/websocket_service.py

@@ -498,6 +498,76 @@ class WebSocketService:
         }
         await self._manager.send_personal_message(message, connection_id)
 
+    async def broadcast_scan_started(self, directory: str) -> None:
+        """Broadcast that a library scan has started.
+        
+        Args:
+            directory: The root directory path being scanned
+        """
+        message = {
+            "type": "scan_started",
+            "timestamp": datetime.now(timezone.utc).isoformat(),
+            "data": {
+                "directory": directory,
+            },
+        }
+        await self._manager.broadcast(message)
+        logger.info("Broadcast scan_started", directory=directory)
+
+    async def broadcast_scan_progress(
+        self,
+        directories_scanned: int,
+        files_found: int,
+        current_directory: str,
+    ) -> None:
+        """Broadcast scan progress update to all clients.
+        
+        Args:
+            directories_scanned: Number of directories scanned so far
+            files_found: Number of MP4 files found so far
+            current_directory: Current directory being scanned
+        """
+        message = {
+            "type": "scan_progress",
+            "timestamp": datetime.now(timezone.utc).isoformat(),
+            "data": {
+                "directories_scanned": directories_scanned,
+                "files_found": files_found,
+                "current_directory": current_directory,
+            },
+        }
+        await self._manager.broadcast(message)
+
+    async def broadcast_scan_completed(
+        self,
+        total_directories: int,
+        total_files: int,
+        elapsed_seconds: float,
+    ) -> None:
+        """Broadcast scan completion to all clients.
+        
+        Args:
+            total_directories: Total number of directories scanned
+            total_files: Total number of MP4 files found
+            elapsed_seconds: Time taken for the scan in seconds
+        """
+        message = {
+            "type": "scan_completed",
+            "timestamp": datetime.now(timezone.utc).isoformat(),
+            "data": {
+                "total_directories": total_directories,
+                "total_files": total_files,
+                "elapsed_seconds": round(elapsed_seconds, 2),
+            },
+        }
+        await self._manager.broadcast(message)
+        logger.info(
+            "Broadcast scan_completed",
+            total_directories=total_directories,
+            total_files=total_files,
+            elapsed_seconds=round(elapsed_seconds, 2),
+        )
+
 
 # Singleton instance for application-wide access
 _websocket_service: Optional[WebSocketService] = None