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/banner_routes.py

@@ -6,7 +6,7 @@ Public SDK-Endpoints (fuer Einbettung) + Admin-Endpoints (Konfiguration & Stats)
 
 import uuid
 import hashlib
-from datetime import datetime, timedelta
+from datetime import datetime, timedelta, timezone
 from typing import Optional, List
 
 from fastapi import APIRouter, Depends, HTTPException, Query, Header
@@ -206,8 +206,8 @@ async def record_consent(
         existing.ip_hash = ip_hash
         existing.user_agent = body.user_agent
         existing.consent_string = body.consent_string
-        existing.expires_at = datetime.utcnow() + timedelta(days=365)
-        existing.updated_at = datetime.utcnow()
+        existing.expires_at = datetime.now(timezone.utc) + timedelta(days=365)
+        existing.updated_at = datetime.now(timezone.utc)
         db.flush()
 
         _log_banner_audit(
@@ -227,7 +227,7 @@ async def record_consent(
         ip_hash=ip_hash,
         user_agent=body.user_agent,
         consent_string=body.consent_string,
-        expires_at=datetime.utcnow() + timedelta(days=365),
+        expires_at=datetime.now(timezone.utc) + timedelta(days=365),
     )
     db.add(consent)
     db.flush()
@@ -476,7 +476,7 @@ async def update_site_config(
         if val is not None:
             setattr(config, field, val)
 
-    config.updated_at = datetime.utcnow()
+    config.updated_at = datetime.now(timezone.utc)
     db.commit()
     db.refresh(config)
     return _site_config_to_dict(config)