fix: restore AsyncSession import and fix dead regex patterns in NOT_FOUND_PATTERNS

- Added missing 'from sqlalchemy.ext.asyncio import AsyncSession' import in downloads.py
  This fixes the NameError at module load time that was breaking all download tasks

- Replaced regex patterns with plain-string equivalents in NOT_FOUND_PATTERNS:
  * 'account.*deleted' → 'account deleted' (. and * are literal in substring search)
  * 'profile.*not.*exist' → 'profile does not exist' (same issue)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/services/gallery_dl.py

@@ -169,8 +169,8 @@ class GalleryDLService:
         "artist not found",
         "content no longer available",
         "has been deleted",
-        "account.*deleted",
-        "profile.*not.*exist",
+        "account deleted",
+        "profile does not exist",
     ]
     GENERIC_NOT_FOUND_PATTERNS = ["does not exist", "no longer available"]
     NETWORK_ERROR_PATTERNS = [