feat(download): persist retry state and dead-letter

Retry count and queue status were in-memory only and lost on
restart, so failed downloads could not be safely resumed and
permanently-failed episodes silently blocked re-queueing via the
episode-id unique index.

- Add status + retry_count columns to DownloadQueueItem
- Replace unique(episode_id) with unique(episode_id, status) so
  permanently_failed rows do not block new pending entries
- Add PERMANENTLY_FAILED to DownloadStatus enum
- Persist retry_count on each failure; mark permanently_failed once
  max_retries reached
- QueueRepository reads status/retry_count from DB instead of
  defaulting to PENDING/0
- Stop double-incrementing retry_count in retry_failed_items;
  increment only happens in _process_download on failure

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

src/server/database/service.py

@@ -748,6 +748,8 @@ class DownloadQueueService:
         episode_id: int,
         download_url: Optional[str] = None,
         file_destination: Optional[str] = None,
+        status: str = "pending",
+        retry_count: int = 0,
     ) -> DownloadQueueItem:
         """Add item to download queue.
         
@@ -757,6 +759,8 @@ class DownloadQueueService:
             episode_id: Foreign key to Episode
             download_url: Optional provider download URL
             file_destination: Optional target file path
+            status: Queue item status (default: "pending")
+            retry_count: Number of retry attempts (default: 0)
         
         Returns:
             Created DownloadQueueItem instance
@@ -766,13 +770,15 @@ class DownloadQueueService:
             episode_id=episode_id,
             download_url=download_url,
             file_destination=file_destination,
+            status=status,
+            retry_count=retry_count,
         )
         db.add(item)
         await db.flush()
         await db.refresh(item)
         logger.info(
             f"Added to download queue: episode_id={episode_id} "
-            f"for series_id={series_id}"
+            f"for series_id={series_id}, status={status}"
         )
         return item
     
@@ -799,21 +805,24 @@ class DownloadQueueService:
     async def get_by_episode(
         db: AsyncSession,
         episode_id: int,
+        status_filter: Optional[str] = None,
     ) -> Optional[DownloadQueueItem]:
         """Get download queue item by episode ID.
         
         Args:
             db: Database session
             episode_id: Foreign key to Episode
+            status_filter: Optional status to filter by (e.g., "pending")
         
         Returns:
             DownloadQueueItem instance or None if not found
         """
-        result = await db.execute(
-            select(DownloadQueueItem).where(
-                DownloadQueueItem.episode_id == episode_id
-            )
+        query = select(DownloadQueueItem).where(
+            DownloadQueueItem.episode_id == episode_id
         )
+        if status_filter:
+            query = query.where(DownloadQueueItem.status == status_filter)
+        result = await db.execute(query)
         return result.scalar_one_or_none()
     
     @staticmethod
@@ -873,6 +882,95 @@ class DownloadQueueService:
         logger.debug("Set error on download queue item %s", item_id)
         return item
     
+    @staticmethod
+    async def set_status(
+        db: AsyncSession,
+        item_id: int,
+        status: str,
+    ) -> Optional[DownloadQueueItem]:
+        """Set status on download queue item.
+        
+        Args:
+            db: Database session
+            item_id: Item primary key
+            status: New status value
+        
+        Returns:
+            Updated DownloadQueueItem instance or None if not found
+        """
+        item = await DownloadQueueService.get_by_id(db, item_id)
+        if not item:
+            return None
+        
+        item.status = status
+        
+        await db.flush()
+        await db.refresh(item)
+        logger.debug("Set status on download queue item %s to %s", item_id, status)
+        return item
+    
+    @staticmethod
+    async def increment_retry_count(
+        db: AsyncSession,
+        item_id: int,
+    ) -> Optional[DownloadQueueItem]:
+        """Increment retry count on download queue item.
+        
+        Args:
+            db: Database session
+            item_id: Item primary key
+        
+        Returns:
+            Updated DownloadQueueItem instance or None if not found
+        """
+        item = await DownloadQueueService.get_by_id(db, item_id)
+        if not item:
+            return None
+        
+        item.retry_count += 1
+        
+        await db.flush()
+        await db.refresh(item)
+        logger.debug(
+            "Incremented retry count on download queue item %s to %s",
+            item_id, item.retry_count
+        )
+        return item
+    
+    @staticmethod
+    async def set_status_and_error(
+        db: AsyncSession,
+        item_id: int,
+        status: str,
+        error_message: Optional[str] = None,
+    ) -> Optional[DownloadQueueItem]:
+        """Set status and error message on download queue item atomically.
+        
+        Args:
+            db: Database session
+            item_id: Item primary key
+            status: New status value
+            error_message: Optional error description
+        
+        Returns:
+            Updated DownloadQueueItem instance or None if not found
+        """
+        item = await DownloadQueueService.get_by_id(db, item_id)
+        if not item:
+            return None
+        
+        item.status = status
+        if error_message is not None:
+            item.error_message = error_message
+        
+        await db.flush()
+        await db.refresh(item)
+        logger.debug(
+            "Set status=%s on download queue item %s, error=%s",
+            status, item_id, error_message
+        )
+        return item
+    
     @staticmethod
     async def delete(db: AsyncSession, item_id: int) -> bool:
         """Delete download queue item.