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/api/vvt_routes.py

@@ -119,7 +119,7 @@ async def upsert_organization(
     else:
         for field, value in request.dict(exclude_none=True).items():
             setattr(org, field, value)
-        org.updated_at = datetime.utcnow()
+        org.updated_at = datetime.now(timezone.utc)
 
     db.commit()
     db.refresh(org)
@@ -291,7 +291,7 @@ async def update_activity(
     updates = request.dict(exclude_none=True)
     for field, value in updates.items():
         setattr(act, field, value)
-    act.updated_at = datetime.utcnow()
+    act.updated_at = datetime.now(timezone.utc)
 
     _log_audit(
         db,
@@ -408,7 +408,7 @@ async def export_activities(
         return _export_csv(activities)
 
     return {
-        "exported_at": datetime.utcnow().isoformat(),
+        "exported_at": datetime.now(timezone.utc).isoformat(),
         "organization": {
             "name": org.organization_name if org else "",
             "dpo_name": org.dpo_name if org else "",
@@ -482,7 +482,7 @@ def _export_csv(activities: list) -> StreamingResponse:
         iter([output.getvalue()]),
         media_type='text/csv; charset=utf-8',
         headers={
-            'Content-Disposition': f'attachment; filename="vvt_export_{datetime.utcnow().strftime("%Y%m%d")}.csv"'
+            'Content-Disposition': f'attachment; filename="vvt_export_{datetime.now(timezone.utc).strftime("%Y%m%d")}.csv"'
         },
     )