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/scripts/test_clean_release_cli.py

@@ -5,21 +5,19 @@
 
 """Smoke tests for the redesigned clean release CLI commands."""
 
+from datetime import UTC, datetime
 from types import SimpleNamespace
-import json
-
-from src.dependencies import get_clean_release_repository, get_config_manager
-from datetime import datetime, timezone
 from uuid import uuid4
 
+from src.dependencies import get_clean_release_repository, get_config_manager
 from src.models.clean_release import (
     CleanPolicySnapshot,
     ComplianceReport,
     ReleaseCandidate,
     SourceRegistrySnapshot,
 )
-from src.services.clean_release.enums import CandidateStatus, ComplianceDecision
 from src.scripts.clean_release_cli import main as cli_main
+from src.services.clean_release.enums import CandidateStatus, ComplianceDecision
 
 
 # [DEF:test_cli_candidate_register_scaffold:Function]
@@ -227,7 +225,7 @@ def test_cli_release_gate_commands_scaffold() -> None:
             version="1.0.0",
             source_snapshot_ref="git:sha-approved",
             created_by="cli-test",
-            created_at=datetime.now(timezone.utc),
+            created_at=datetime.now(UTC),
             status=CandidateStatus.CHECK_PASSED.value,
         )
     )
@@ -237,7 +235,7 @@ def test_cli_release_gate_commands_scaffold() -> None:
             version="1.0.0",
             source_snapshot_ref="git:sha-rejected",
             created_by="cli-test",
-            created_at=datetime.now(timezone.utc),
+            created_at=datetime.now(UTC),
             status=CandidateStatus.CHECK_PASSED.value,
         )
     )
@@ -252,7 +250,7 @@ def test_cli_release_gate_commands_scaffold() -> None:
                 "violations_count": 0,
                 "blocking_violations_count": 0,
             },
-            generated_at=datetime.now(timezone.utc),
+            generated_at=datetime.now(UTC),
             immutable=True,
         )
     )
@@ -267,7 +265,7 @@ def test_cli_release_gate_commands_scaffold() -> None:
                 "violations_count": 0,
                 "blocking_violations_count": 0,
             },
-            generated_at=datetime.now(timezone.utc),
+            generated_at=datetime.now(UTC),
             immutable=True,
         )
     )