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/tests/test_audit_routes.py

@@ -8,7 +8,7 @@ Run with: pytest backend/compliance/tests/test_audit_routes.py -v
 
 import pytest
 import hashlib
-from datetime import datetime
+from datetime import datetime, timezone
 from unittest.mock import MagicMock
 from uuid import uuid4
 
@@ -78,7 +78,7 @@ def sample_session():
         completed_items=0,
         compliant_count=0,
         non_compliant_count=0,
-        created_at=datetime.utcnow(),
+        created_at=datetime.now(timezone.utc),
     )
 
 
@@ -94,7 +94,7 @@ def sample_signoff(sample_session, sample_requirement):
         signature_hash=None,
         signed_at=None,
         signed_by=None,
-        created_at=datetime.utcnow(),
+        created_at=datetime.now(timezone.utc),
     )
 
 
@@ -214,7 +214,7 @@ class TestAuditSessionLifecycle:
         assert sample_session.status == AuditSessionStatusEnum.DRAFT
 
         sample_session.status = AuditSessionStatusEnum.IN_PROGRESS
-        sample_session.started_at = datetime.utcnow()
+        sample_session.started_at = datetime.now(timezone.utc)
 
         assert sample_session.status == AuditSessionStatusEnum.IN_PROGRESS
         assert sample_session.started_at is not None
@@ -231,7 +231,7 @@ class TestAuditSessionLifecycle:
         sample_session.status = AuditSessionStatusEnum.IN_PROGRESS
 
         sample_session.status = AuditSessionStatusEnum.COMPLETED
-        sample_session.completed_at = datetime.utcnow()
+        sample_session.completed_at = datetime.now(timezone.utc)
 
         assert sample_session.status == AuditSessionStatusEnum.COMPLETED
         assert sample_session.completed_at is not None
@@ -353,7 +353,7 @@ class TestSignOff:
     def test_signoff_with_signature_creates_hash(self, sample_session, sample_requirement):
         """Signing off with signature should create SHA-256 hash."""
         result = AuditResultEnum.COMPLIANT
-        timestamp = datetime.utcnow().isoformat()
+        timestamp = datetime.now(timezone.utc).isoformat()
         data = f"{result.value}|{sample_requirement.id}|{sample_session.auditor_name}|{timestamp}"
         signature_hash = hashlib.sha256(data.encode()).hexdigest()
 
@@ -382,7 +382,7 @@ class TestSignOff:
 
         # First sign-off should trigger auto-start
         sample_session.status = AuditSessionStatusEnum.IN_PROGRESS
-        sample_session.started_at = datetime.utcnow()
+        sample_session.started_at = datetime.now(timezone.utc)
 
         assert sample_session.status == AuditSessionStatusEnum.IN_PROGRESS
 
@@ -390,7 +390,7 @@ class TestSignOff:
         """Updating an existing sign-off should work."""
         sample_signoff.result = AuditResultEnum.NON_COMPLIANT
         sample_signoff.notes = "Updated: needs improvement"
-        sample_signoff.updated_at = datetime.utcnow()
+        sample_signoff.updated_at = datetime.now(timezone.utc)
 
         assert sample_signoff.result == AuditResultEnum.NON_COMPLIANT
         assert "Updated" in sample_signoff.notes
@@ -423,7 +423,7 @@ class TestGetSignOff:
 
         # With signature
         sample_signoff.signature_hash = "abc123"
-        sample_signoff.signed_at = datetime.utcnow()
+        sample_signoff.signed_at = datetime.now(timezone.utc)
         sample_signoff.signed_by = "Test Auditor"
 
         assert sample_signoff.signature_hash == "abc123"