Task 4.4: Update WebSocket API Endpoints to use key identifier

- Updated src/server/api/websocket.py docstrings to document key as primary series identifier
- Updated src/server/models/websocket.py with detailed docstrings explaining key and folder fields in message payloads
- Updated src/server/services/websocket_service.py broadcast method docstrings to document key field usage
- Added WebSocket message example with key in infrastructure.md
- All 83 WebSocket tests pass
- Task 4.4 marked as complete in instructions.md

src/server/api/websocket.py

@@ -2,6 +2,14 @@
 
 This module provides WebSocket endpoints for clients to connect and receive
 real-time updates about downloads, queue status, and system events.
+
+Series Identifier Convention:
+    - `key`: Primary identifier for series (provider-assigned, URL-safe)
+             e.g., "attack-on-titan"
+    - `folder`: Display metadata only (e.g., "Attack on Titan (2013)")
+    
+All series-related WebSocket events include `key` as the primary identifier
+in their data payload. The `folder` field is optional for display purposes.
 """
 from __future__ import annotations
 
@@ -58,19 +66,25 @@ async def websocket_endpoint(
     }
     ```
     
-    Server message format:
+    Server message format (series-related events include 'key' identifier):
     ```json
     {
         "type": "download_progress",
         "timestamp": "2025-10-17T10:30:00.000Z",
         "data": {
             "download_id": "abc123",
+            "key": "attack-on-titan",
+            "folder": "Attack on Titan (2013)",
             "percent": 45.2,
             "speed_mbps": 2.5,
             "eta_seconds": 180
         }
     }
     ```
+    
+    Note:
+        - `key` is the primary series identifier (provider-assigned, URL-safe)
+        - `folder` is optional display metadata
     """
     connection_id = str(uuid.uuid4())
     user_id: Optional[str] = None