fix: parse string-encoded JSON in episode fields + return success for empty queue

- Add field validators to parse season/episode/title from JSON strings
- Add episodes list parser handling string-encoded dicts
- Return 200 with message instead of 400 when queue empty
- Remove completed tasks 8+9 from Docs

src/server/api/download.py

@@ -339,9 +339,10 @@ async def start_queue(
         result = await download_service.start_queue_processing()
         
         if result is None:
-            raise BadRequestError(
-                message="No pending downloads in queue"
-            )
+            return {
+                "status": "success",
+                "message": "No pending downloads in queue",
+            }
 
         return {
             "status": "success",