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/models.py

@@ -316,6 +316,7 @@ class DownloadQueueItem(Base, TimestampMixin):
         id: Primary key
         series_id: Foreign key to AnimeSeries
         episode_id: Foreign key to Episode
+        status: Queue status (pending/downloading/completed/failed/permanently_failed)
         error_message: Error description if failed
         download_url: Provider download URL
         file_destination: Target file path
@@ -347,12 +348,29 @@ class DownloadQueueItem(Base, TimestampMixin):
         index=True
     )
     
-    # Unique constraint to prevent duplicate pending queue items
-    # An episode can only have one queue entry at a time
+    # Status column to track queue item state
+    # Allows distinguishing pending items from permanently failed ones
+    status: Mapped[str] = mapped_column(
+        String(50), nullable=False, default="pending",
+        doc="Queue item status: pending, downloading, completed, failed, permanently_failed"
+    )
+    
+    # Retry count to track failed download attempts
+    # Used to determine when to move item to permanently_failed
+    retry_count: Mapped[int] = mapped_column(
+        Integer, nullable=False, default=0,
+        doc="Number of retry attempts for this download"
+    )
+    
+    # Unique constraint to prevent duplicate pending queue items per episode
+    # An episode can only have one PENDING entry at a time
+    # The status column allows failed items to remain in DB while new
+    # pending items can be added (application-level dedup still required)
     __table_args__ = (
         Index(
-            "ix_download_queue_episode_pending",
+            "ix_download_queue_episode_status",
             "episode_id",
+            "status",
             unique=True,
         ),
     )