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/api/documents.py

@@ -346,7 +346,7 @@ async def confirm_upload(
             "  AND (used_bytes + :delta) <= limit_bytes "
             "RETURNING used_bytes, limit_bytes"
         ),
-        {"delta": size, "uid": str(doc.user_id)},
+        {"delta": size, "uid": doc.user_id.hex},
     )
     row = result.fetchone()
 
@@ -354,7 +354,7 @@ async def confirm_upload(
         # Quota exceeded — fetch current quota state for the 413 body
         quota_result = await session.execute(
             text("SELECT used_bytes, limit_bytes FROM quotas WHERE user_id = :uid"),
-            {"uid": str(doc.user_id)},
+            {"uid": doc.user_id.hex},
         )
         q = quota_result.fetchone()
         # Delete the pending Document row and best-effort remove the MinIO object

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)

backend/tests/test_quota.py

@@ -192,7 +192,6 @@ async def test_quota_exceeded_response(
     assert detail["limit_bytes"] == 104_857_600, f"Unexpected limit_bytes: {detail}"
 
 
-@pytest.mark.xfail(strict=False, reason="requires PostgreSQL for atomic UUID-typed quota SQL")
 async def test_delete_decrements_quota(
     async_client, db_session, auth_user, mock_minio_presigned, mock_minio_stat, monkeypatch
 ):