feat: graceful shutdown and WAL cleanup

- Add stop_grace_period to backend container for graceful shutdown
- Document WAL mode rationale and orphaned file cleanup in db.py
- Handle database close errors gracefully in lifespan
- Clean up orphaned WAL files during startup before opening DB
- Reorder imports and fix formatting in startup.py

backend/app/db.py

@@ -274,7 +274,18 @@ CREATE INDEX IF NOT EXISTS idx_import_log_source_id_desc
 
 
 async def _configure_connection(db: aiosqlite.Connection) -> None:
-    """Apply hardening pragmas to a newly-opened SQLite connection."""
+    """Apply hardening pragmas to a newly-opened SQLite connection.
+
+    WAL mode is intentionally kept despite the risk of orphaned ``.wal``/``.shm``
+    files after unclean shutdowns.  The benefits for concurrent readers
+    (readers do not block writers) outweigh the cleanup overhead, especially
+    under load.  BanGUI runs as a single worker, but multiple concurrent HTTP
+    requests can still issue overlapping reads; DELETE mode would serialize
+    those reads behind any write, degrading API performance.
+
+    Orphaned files are handled by :func:`_cleanup_wal_files`, which is called
+    during startup before the database is opened.
+    """
     await db.execute("PRAGMA journal_mode=WAL;")
     await db.execute("PRAGMA foreign_keys=ON;")
     await db.execute("PRAGMA busy_timeout=5000;")