feat: implement WebSocket real-time progress updates

- Add ProgressService for centralized progress tracking and broadcasting
- Integrate ProgressService with DownloadService for download progress
- Integrate ProgressService with AnimeService for scan progress
- Add progress-related WebSocket message models (ScanProgress, ErrorNotification, etc.)
- Initialize ProgressService with WebSocket callback in application startup
- Add comprehensive unit tests for ProgressService
- Update infrastructure.md with ProgressService documentation
- Remove completed WebSocket Real-time Updates task from instructions.md

The ProgressService provides:
- Real-time progress tracking for downloads, scans, and queue operations
- Automatic progress percentage calculation
- Progress lifecycle management (start, update, complete, fail, cancel)
- WebSocket integration for instant client updates
- Progress history with size limits
- Thread-safe operations using asyncio locks
- Support for metadata and custom messages

Benefits:
- Decoupled progress tracking from WebSocket broadcasting
- Single reusable service across all components
- Supports multiple concurrent operations efficiently
- Centralized progress tracking simplifies monitoring
- Instant feedback to users on long-running operations

src/server/models/websocket.py

@@ -30,6 +30,11 @@ class WebSocketMessageType(str, Enum):
     QUEUE_PAUSED = "queue_paused"
     QUEUE_RESUMED = "queue_resumed"
     
+    # Progress-related messages
+    SCAN_PROGRESS = "scan_progress"
+    SCAN_COMPLETE = "scan_complete"
+    SCAN_FAILED = "scan_failed"
+    
     # System messages
     SYSTEM_INFO = "system_info"
     SYSTEM_WARNING = "system_warning"
@@ -188,3 +193,93 @@ class RoomSubscriptionRequest(BaseModel):
     room: str = Field(
         ..., min_length=1, description="Room name to join or leave"
     )
+
+
+class ScanProgressMessage(BaseModel):
+    """Scan progress update message."""
+
+    type: WebSocketMessageType = Field(
+        default=WebSocketMessageType.SCAN_PROGRESS,
+        description="Message type",
+    )
+    timestamp: str = Field(
+        default_factory=lambda: datetime.utcnow().isoformat(),
+        description="ISO 8601 timestamp",
+    )
+    data: Dict[str, Any] = Field(
+        ...,
+        description="Scan progress data including current, total, percent",
+    )
+
+
+class ScanCompleteMessage(BaseModel):
+    """Scan completion message."""
+
+    type: WebSocketMessageType = Field(
+        default=WebSocketMessageType.SCAN_COMPLETE,
+        description="Message type",
+    )
+    timestamp: str = Field(
+        default_factory=lambda: datetime.utcnow().isoformat(),
+        description="ISO 8601 timestamp",
+    )
+    data: Dict[str, Any] = Field(
+        ...,
+        description="Scan completion data including series_found, duration",
+    )
+
+
+class ScanFailedMessage(BaseModel):
+    """Scan failure message."""
+
+    type: WebSocketMessageType = Field(
+        default=WebSocketMessageType.SCAN_FAILED,
+        description="Message type",
+    )
+    timestamp: str = Field(
+        default_factory=lambda: datetime.utcnow().isoformat(),
+        description="ISO 8601 timestamp",
+    )
+    data: Dict[str, Any] = Field(
+        ..., description="Scan error data including error_message"
+    )
+
+
+class ErrorNotificationMessage(BaseModel):
+    """Error notification message for critical errors."""
+
+    type: WebSocketMessageType = Field(
+        default=WebSocketMessageType.SYSTEM_ERROR,
+        description="Message type",
+    )
+    timestamp: str = Field(
+        default_factory=lambda: datetime.utcnow().isoformat(),
+        description="ISO 8601 timestamp",
+    )
+    data: Dict[str, Any] = Field(
+        ...,
+        description=(
+            "Error notification data including severity, message, details"
+        ),
+    )
+
+
+class ProgressUpdateMessage(BaseModel):
+    """Generic progress update message.
+    
+    Can be used for any type of progress (download, scan, queue, etc.)
+    """
+
+    type: WebSocketMessageType = Field(
+        ..., description="Type of progress message"
+    )
+    timestamp: str = Field(
+        default_factory=lambda: datetime.utcnow().isoformat(),
+        description="ISO 8601 timestamp",
+    )
+    data: Dict[str, Any] = Field(
+        ...,
+        description=(
+            "Progress data including id, status, percent, current, total"
+        ),
+    )