chore(lint): apply ruff --fix (4443 auto-fixes)

Auto-fixed categories:
- F401: unused imports removed
- I001: import blocks sorted
- W293: trailing whitespace stripped
- UP035: deprecated typing imports replaced
- SIM: simplify suggestions applied
- ARG: unused args prefixed with underscore
- T201: print statements removed
- F841: unused variables removed
- RUF059: unpacked variables prefixed

Remaining ~1500 unfixable errors (C901, B904, N806, E402) require manual work.

Backend smoke tests: 13/13 passed.

backend/tests/services/clean_release/test_candidate_manifest_services.py

@@ -3,18 +3,19 @@
 # @PURPOSE: Test lifecycle and manifest versioning for release candidates.
 # @LAYER: Tests
 
+from datetime import UTC, datetime
+
 import pytest
-from datetime import datetime, timezone
 from sqlalchemy import create_engine
 from sqlalchemy.orm import sessionmaker
+
 from src.core.database import Base
 from src.models.clean_release import (
-    ReleaseCandidate,
     DistributionManifest,
-    CandidateArtifact,
+    ReleaseCandidate,
 )
-from src.services.clean_release.enums import CandidateStatus
 from src.services.clean_release.candidate_service import register_candidate
+from src.services.clean_release.enums import CandidateStatus
 from src.services.clean_release.manifest_service import build_manifest_snapshot
 from src.services.clean_release.repository import CleanReleaseRepository
 
@@ -79,7 +80,7 @@ def test_manifest_versioning_and_immutability(db_session):
         artifacts_digest="hash1",
         source_snapshot_ref="ref1",
         content_json={},
-        created_at=datetime.now(timezone.utc),
+        created_at=datetime.now(UTC),
         created_by="operator",
     )
     db_session.add(m1)
@@ -93,7 +94,7 @@ def test_manifest_versioning_and_immutability(db_session):
         artifacts_digest="hash2",
         source_snapshot_ref="ref1",
         content_json={},
-        created_at=datetime.now(timezone.utc),
+        created_at=datetime.now(UTC),
         created_by="operator",
     )
     db_session.add(m2)