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/source_policy_router.py

@@ -21,11 +21,11 @@ Endpoints:
   GET    /api/v1/admin/compliance-report    — Compliance report
 """
 
-from datetime import datetime
+from datetime import datetime, timezone
 from typing import Optional
 
 from fastapi import APIRouter, HTTPException, Depends, Query
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, ConfigDict, Field
 from sqlalchemy.orm import Session
 
 from database import get_db
@@ -83,8 +83,7 @@ class SourceResponse(BaseModel):
     created_at: str
     updated_at: Optional[str] = None
 
-    class Config:
-        from_attributes = True
+    model_config = ConfigDict(from_attributes=True)
 
 
 class OperationUpdate(BaseModel):
@@ -530,7 +529,7 @@ async def get_policy_stats(db: Session = Depends(get_db)):
     pii_rules = db.query(PIIRuleDB).filter(PIIRuleDB.active).count()
 
     # Count blocked content entries from today
-    today_start = datetime.utcnow().replace(hour=0, minute=0, second=0, microsecond=0)
+    today_start = datetime.now(timezone.utc).replace(hour=0, minute=0, second=0, microsecond=0)
     blocked_today = db.query(BlockedContentDB).filter(
         BlockedContentDB.created_at >= today_start,
     ).count()
@@ -553,7 +552,7 @@ async def get_compliance_report(db: Session = Depends(get_db)):
     pii_rules = db.query(PIIRuleDB).filter(PIIRuleDB.active).all()
 
     return {
-        "report_date": datetime.utcnow().isoformat(),
+        "report_date": datetime.now(timezone.utc).isoformat(),
         "summary": {
             "active_sources": len(sources),
             "active_pii_rules": len(pii_rules),