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/auto_risk_updater.py

@@ -11,7 +11,7 @@ Sprint 6: CI/CD Evidence Collection (2026-01-18)
 """
 
 import logging
-from datetime import datetime
+from datetime import datetime, timezone
 from typing import Dict, List, Optional
 from dataclasses import dataclass
 from enum import Enum
@@ -140,7 +140,7 @@ class AutoRiskUpdater:
         if new_status != old_status:
             control.status = ControlStatusEnum(new_status)
             control.status_notes = self._generate_status_notes(scan_result)
-            control.updated_at = datetime.utcnow()
+            control.updated_at = datetime.now(timezone.utc)
             control_updated = True
             logger.info(f"Control {scan_result.control_id} status changed: {old_status} -> {new_status}")
 
@@ -225,7 +225,7 @@ class AutoRiskUpdater:
             source="ci_pipeline",
             ci_job_id=scan_result.ci_job_id,
             status=EvidenceStatusEnum.VALID,
-            valid_from=datetime.utcnow(),
+            valid_from=datetime.now(timezone.utc),
             collected_at=scan_result.timestamp,
         )
 
@@ -298,8 +298,8 @@ class AutoRiskUpdater:
                         risk_updated = True
 
             if risk_updated:
-                risk.last_assessed_at = datetime.utcnow()
-                risk.updated_at = datetime.utcnow()
+                risk.last_assessed_at = datetime.now(timezone.utc)
+                risk.updated_at = datetime.now(timezone.utc)
                 affected_risks.append(risk.risk_id)
                 logger.info(f"Updated risk {risk.risk_id} due to control {control.control_id} status change")
 
@@ -354,7 +354,7 @@ class AutoRiskUpdater:
         try:
             ts = datetime.fromisoformat(timestamp.replace('Z', '+00:00'))
         except (ValueError, AttributeError):
-            ts = datetime.utcnow()
+            ts = datetime.now(timezone.utc)
 
         # Determine scan type from evidence_type
         scan_type = ScanType.SAST  # Default