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/services/report_generator.py

@@ -11,7 +11,7 @@ Reports include:
 """
 
 import logging
-from datetime import datetime, date, timedelta
+from datetime import datetime, date, timedelta, timezone
 from typing import Dict, List, Any, Optional
 from enum import Enum
 
@@ -75,7 +75,7 @@ class ComplianceReportGenerator:
 
         report = {
             "report_metadata": {
-                "generated_at": datetime.utcnow().isoformat(),
+                "generated_at": datetime.now(timezone.utc).isoformat(),
                 "period": period.value,
                 "as_of_date": as_of_date.isoformat(),
                 "date_range_start": date_range["start"].isoformat(),
@@ -415,7 +415,7 @@ class ComplianceReportGenerator:
         evidence_stats = self.evidence_repo.get_statistics()
 
         return {
-            "generated_at": datetime.utcnow().isoformat(),
+            "generated_at": datetime.now(timezone.utc).isoformat(),
             "compliance_score": stats.get("compliance_score", 0),
             "controls": {
                 "total": stats.get("total", 0),