chore(backend): deprecation sweep — Pydantic V1 -> V2, utcnow -> tz-aware

Two low-risk Pydantic V1 idioms that will be hard errors in V3:
  - Query(regex=...) -> Query(pattern=...) (audit_routes, control_generator_routes)
  - class Config: from_attributes=True -> model_config = ConfigDict(...)
    in source_policy_router.py (schemas.py is intentionally skipped — it is
    the Phase 1 schema-split target and the ConfigDict conversion is most
    efficient to do during that split).

Naive -> aware datetime sweep across 47 files:
  - datetime.utcnow() -> datetime.now(timezone.utc)
  - default=datetime.utcnow -> default=lambda: datetime.now(timezone.utc)
  - onupdate=datetime.utcnow -> onupdate=lambda: datetime.now(timezone.utc)

All SQLAlchemy DateTime columns in the project already declare
timezone=True, so the DB schema expects aware datetimes. Before this
commit, the in-Python side was generating naive values and the driver
was silently coercing them. This is a latent-bug fix, not a behavior
change at the DB boundary.

Verified:
  - 173/173 pytest compliance/tests/ pass (same as baseline)
  - tests/contracts/test_openapi_baseline.py passes (360 paths,
    484 operations unchanged)
  - DeprecationWarning count dropped from 158 -> 35

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend-compliance/compliance/services/export_generator.py

@@ -16,7 +16,7 @@ import os
 import shutil
 import tempfile
 import zipfile
-from datetime import datetime, date
+from datetime import datetime, date, timezone
 from pathlib import Path
 from typing import Dict, List, Optional, Any
 
@@ -98,7 +98,7 @@ class AuditExportGenerator:
             export_record.file_hash = file_hash
             export_record.file_size_bytes = file_size
             export_record.status = ExportStatusEnum.COMPLETED
-            export_record.completed_at = datetime.utcnow()
+            export_record.completed_at = datetime.now(timezone.utc)
 
             # Calculate statistics
             stats = self._calculate_statistics(