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/escalation_routes.py

@@ -12,7 +12,7 @@ Endpoints:
 """
 
 import logging
-from datetime import datetime
+from datetime import datetime, timezone
 from typing import Optional, Any, Dict
 
 from fastapi import APIRouter, Depends, HTTPException, Query, Header
@@ -244,7 +244,7 @@ async def update_escalation(
 
     set_clauses = ", ".join(f"{k} = :{k}" for k in updates.keys())
     updates["id"] = escalation_id
-    updates["updated_at"] = datetime.utcnow()
+    updates["updated_at"] = datetime.now(timezone.utc)
 
     row = db.execute(
         text(
@@ -277,7 +277,7 @@ async def update_status(
 
     resolved_at = request.resolved_at
     if request.status in ('resolved', 'closed') and resolved_at is None:
-        resolved_at = datetime.utcnow()
+        resolved_at = datetime.now(timezone.utc)
 
     row = db.execute(
         text(
@@ -288,7 +288,7 @@ async def update_status(
         {
             "status": request.status,
             "resolved_at": resolved_at,
-            "updated_at": datetime.utcnow(),
+            "updated_at": datetime.now(timezone.utc),
             "id": escalation_id,
         },
     ).fetchone()