Add queue deduplication to prevent duplicate entries

- In-memory dedup in add_to_queue() using _pending_by_episode dict
- Batch-local dedup via seen_in_batch set (handles duplicates within single call)
- Database unique index on episode_id via __table_args__
- 5-minute cooldown in _auto_download_missing() to prevent rapid re-triggers
- Updated _add_to_pending_queue() and _remove_from_pending_queue() to track episode keys
- Added TestQueueDeduplication with 4 test cases
- Updated DEVELOPMENT.md and TESTING.md with queue dedup docs

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/server/services/download_service.py

@@ -79,6 +79,9 @@ class DownloadService:
         self._pending_queue: deque[DownloadItem] = deque()
         # Helper dict for O(1) lookup of pending items by ID
         self._pending_items_by_id: Dict[str, DownloadItem] = {}
+        # Helper dict for O(1) lookup of pending items by episode identity
+        # Key: (serie_id, season, episode), Value: item ID
+        self._pending_by_episode: Dict[tuple, str] = {}
         self._active_download: Optional[DownloadItem] = None
         self._completed_items: deque[DownloadItem] = deque(maxlen=100)
         self._failed_items: deque[DownloadItem] = deque(maxlen=50)
@@ -409,7 +412,7 @@ class DownloadService:
     def _add_to_pending_queue(
         self, item: DownloadItem, front: bool = False
     ) -> None:
-        """Add item to pending queue and update helper dict.
+        """Add item to pending queue and update helper dicts.
         
         Args:
             item: Download item to add
@@ -420,9 +423,12 @@ class DownloadService:
         else:
             self._pending_queue.append(item)
         self._pending_items_by_id[item.id] = item
+        # Track by episode identity for deduplication
+        ep_key = (item.serie_id, item.episode.season, item.episode.episode)
+        self._pending_by_episode[ep_key] = item.id
 
     def _remove_from_pending_queue(self, item_or_id: str) -> Optional[DownloadItem]:  # noqa: E501
-        """Remove item from pending queue and update helper dict.
+        """Remove item from pending queue and update helper dicts.
         
         Args:
             item_or_id: Item ID to remove
@@ -442,6 +448,10 @@ class DownloadService:
         try:
             self._pending_queue.remove(item)
             del self._pending_items_by_id[item_id]
+            # Clean up episode tracking
+            ep_key = (item.serie_id, item.episode.season, item.episode.episode)
+            if self._pending_by_episode.get(ep_key) == item_id:
+                del self._pending_by_episode[ep_key]
             return item
         except (ValueError, KeyError):
             return None
@@ -481,10 +491,35 @@ class DownloadService:
         # Initialize queue progress tracking if not already done
         await self._init_queue_progress()
         
+        # Filter out episodes already in pending queue
+        episodes_to_add = []
+        skipped_count = 0
+        seen_in_batch: set = set()  # Track duplicates within this batch
+        for ep in episodes:
+            ep_key = (serie_id, ep.season, ep.episode)
+            if ep_key in self._pending_by_episode or ep_key in seen_in_batch:
+                logger.debug(
+                    "Skipping duplicate episode in queue",
+                    serie_key=serie_id,
+                    season=ep.season,
+                    episode=ep.episode,
+                )
+                skipped_count += 1
+                continue
+            seen_in_batch.add(ep_key)
+            episodes_to_add.append(ep)
+        
+        if skipped_count > 0:
+            logger.info(
+                "Skipped %d duplicate episodes in queue",
+                skipped_count,
+                serie_key=serie_id,
+            )
+        
         created_ids = []
         
         try:
-            for episode in episodes:
+            for episode in episodes_to_add:
                 item = DownloadItem(
                     id=self._generate_item_id(),
                     serie_id=serie_id,