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/services/queue_repository.py

@@ -83,15 +83,12 @@ class QueueRepository:
     ) -> DownloadItem:
         """Convert database model to DownloadItem.
 
-        Note: Since the database model is simplified, status, priority,
-        progress, and retry_count default to initial values.
-
         Args:
             db_item: SQLAlchemy download queue item
             item_id: Optional override for item ID
 
         Returns:
-            Pydantic download item with default status/priority
+            Pydantic download item with status/retry_count from database
         """
         # Get episode info from the related Episode object
         episode = db_item.episode
@@ -109,14 +106,14 @@ class QueueRepository:
             serie_folder=series.folder if series else "",
             serie_name=series.name if series else "",
             episode=episode_identifier,
-            status=DownloadStatus.PENDING,  # Default - managed in-memory
-            priority=DownloadPriority.NORMAL,  # Default - managed in-memory
+            status=DownloadStatus(db_item.status),  # From database
+            priority=DownloadPriority.NORMAL,  # Managed in-memory
             added_at=db_item.created_at or datetime.now(timezone.utc),
             started_at=db_item.started_at,
             completed_at=db_item.completed_at,
             progress=None,  # Managed in-memory
             error=db_item.error_message,
-            retry_count=0,  # Managed in-memory
+            retry_count=db_item.retry_count,  # From database
             source_url=db_item.download_url,
         )
 
@@ -350,6 +347,110 @@ class QueueRepository:
         finally:
             if manage_session:
                 await session.close()
+    
+    async def set_status(
+        self,
+        item_id: str,
+        status: str,
+        db: Optional[AsyncSession] = None,
+    ) -> bool:
+        """Set status on a download item.
+
+        Args:
+            item_id: Download item ID
+            status: New status value
+            db: Optional existing database session
+
+        Returns:
+            True if update succeeded, False if item not found
+
+        Raises:
+            QueueRepositoryError: If update fails
+        """
+        session = db or self._db_session_factory()
+        manage_session = db is None
+
+        try:
+            result = await DownloadQueueService.set_status(
+                session,
+                int(item_id),
+                status,
+            )
+
+            if manage_session:
+                await session.commit()
+
+            success = result is not None
+
+            if success:
+                logger.debug(
+                    "Set status on queue item: item_id=%s, status=%s",
+                    item_id,
+                    status,
+                )
+
+            return success
+
+        except ValueError:
+            return False
+        except Exception as e:
+            if manage_session:
+                await session.rollback()
+            logger.error("Failed to set status: %s", e)
+            raise QueueRepositoryError(f"Failed to set status: {e}") from e
+        finally:
+            if manage_session:
+                await session.close()
+    
+    async def increment_retry(
+        self,
+        item_id: str,
+        db: Optional[AsyncSession] = None,
+    ) -> bool:
+        """Increment retry count on a download item.
+
+        Args:
+            item_id: Download item ID
+            db: Optional existing database session
+
+        Returns:
+            True if update succeeded, False if item not found
+
+        Raises:
+            QueueRepositoryError: If update fails
+        """
+        session = db or self._db_session_factory()
+        manage_session = db is None
+
+        try:
+            result = await DownloadQueueService.increment_retry_count(
+                session,
+                int(item_id),
+            )
+
+            if manage_session:
+                await session.commit()
+
+            success = result is not None
+
+            if success:
+                logger.debug(
+                    "Incremented retry count on queue item: item_id=%s",
+                    item_id,
+                )
+
+            return success
+
+        except ValueError:
+            return False
+        except Exception as e:
+            if manage_session:
+                await session.rollback()
+            logger.error("Failed to increment retry: %s", e)
+            raise QueueRepositoryError(f"Failed to increment retry: {e}") from e
+        finally:
+            if manage_session:
+                await session.close()
 
     async def delete_item(
         self,