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/tests/test_vvt_tenant_isolation.py

@@ -10,7 +10,7 @@ Verifies that:
 import pytest
 import uuid
 from unittest.mock import MagicMock, AsyncMock, patch
-from datetime import datetime
+from datetime import datetime, timezone
 
 from fastapi import HTTPException
 from fastapi.testclient import TestClient
@@ -144,8 +144,8 @@ def _make_activity(tenant_id, vvt_id="VVT-001", name="Test", **kwargs):
     act.next_review_at = None
     act.created_by = "system"
     act.dsfa_id = None
-    act.created_at = datetime.utcnow()
-    act.updated_at = datetime.utcnow()
+    act.created_at = datetime.now(timezone.utc)
+    act.updated_at = datetime.now(timezone.utc)
     return act