fix(phase-03): use UUID.hex in raw SQL to fix SQLite UUID format mismatch

str(uuid_obj) produces dashed 36-char format; SQLite stores UUID as 32-char
hex without dashes, so WHERE user_id = :uid never matched. Using .hex fixes
confirm_upload (api/documents.py) and delete_document (services/storage.py).
Removes stale xfail from test_delete_decrements_quota — now passes on SQLite.

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

backend/services/storage.py

@@ -185,7 +185,7 @@ async def delete_document(
                 "SET used_bytes = CASE WHEN used_bytes > :delta THEN used_bytes - :delta ELSE 0 END "
                 "WHERE user_id = :uid"
             ),
-            {"delta": doc.size_bytes, "uid": str(doc.user_id)},
+            {"delta": doc.size_bytes, "uid": doc.user_id.hex},
         )
 
     await session.delete(doc)