chore: eliminate all deprecation warnings from tests and linter

Warnings fixed:
- datetime.utcnow() → datetime.now(UTC) across 48+ files (src/ + tests/)
- datetime.utcnow (callback ref) → lambda: datetime.now(UTC) in model fields (18 files)
- Pydantic class Config → model_config = ConfigDict(...) (16 files)
- Pydantic .dict() → .model_dump() (8 files)
- ConfigDict(allow_population_by_field_name=True) → validate_by_name=True
- SQLAlchemy declarative_base() import path updated
- FastAPI on_event → lifespan context manager (app.py)
- Import sorting (ruff I001) auto-fixed across all files
- Fixed broken re-export chains that ruff F401 cleanup broke:
  _validate_bcp47: service.py now imports from dictionary_validation directly
  job_to_response: _job_routes.py and test imports from service_utils directly
  fetch_datasource_metadata: restored re-export in service.py
- Added missing TranslateJobService import in _job_routes.py (was deleted by F401)
- Added ConfigDict(protected_namespaces=()) for DashboardDatasetItem schema field
- pytest.ini: replaced deprecated importmode with asyncio_mode

All 440 tests pass with zero deprecation warnings.

backend/tests/test_api_key_routes.py

@@ -7,13 +7,10 @@
 # @TEST_EDGE: revoked_key_delete -> 404
 # @TEST_EDGE: missing_name -> 422
 # @TEST_EDGE: missing_permissions -> 422
-import json
-import sys
-from datetime import datetime, timezone, timedelta
-from unittest.mock import MagicMock, patch
-
 import pytest
-from fastapi import status
+import sys
+from unittest.mock import MagicMock
+
 from fastapi.testclient import TestClient
 
 # Patch GitService at module level to prevent /app/storage/repositories error
@@ -34,8 +31,8 @@ def client():
 
 def _mock_admin_auth():
     """Apply dependency overrides for admin authentication."""
-    from src.dependencies import get_current_user
     from src.app import app
+    from src.dependencies import get_current_user
 
     mock_role = MagicMock()
     mock_role.name = "Admin"
@@ -69,9 +66,10 @@ def mock_db():
     from sqlalchemy import create_engine
     from sqlalchemy.orm import sessionmaker
     from sqlalchemy.pool import StaticPool
-    from src.models.mapping import Base
-    from src.dependencies import get_db
+
     from src.app import app
+    from src.dependencies import get_db
+    from src.models.mapping import Base
 
     engine = create_engine(
         "sqlite:///:memory:",
@@ -113,8 +111,8 @@ class TestListApiKeys:
     # #region test_list_with_keys [C:2] [TYPE Function]
     # @BRIEF Keys exist → returns list without raw_key or key_hash fields.
     def test_list_with_keys(self, client, mock_db):
-        from src.models.api_key import APIKey
         from src.core.auth.api_key import generate_api_key
+        from src.models.api_key import APIKey
 
         # Create a key
         raw, prefix, key_hash = generate_api_key()
@@ -203,8 +201,8 @@ class TestRevokeApiKey:
     # #region test_revoke_valid [C:2] [TYPE Function]
     # @BRIEF Revoke existing active key → 200 with status=revoked.
     def test_revoke_valid(self, client, mock_db):
-        from src.models.api_key import APIKey
         from src.core.auth.api_key import generate_api_key
+        from src.models.api_key import APIKey
 
         raw, prefix, key_hash = generate_api_key()
         api_key = APIKey(
@@ -231,8 +229,8 @@ class TestRevokeApiKey:
     # #region test_revoke_already_revoked [C:2] [TYPE Function]
     # @BRIEF Revoke already revoked key → 404.
     def test_revoke_already_revoked(self, client, mock_db):
-        from src.models.api_key import APIKey
         from src.core.auth.api_key import generate_api_key
+        from src.models.api_key import APIKey
 
         raw, prefix, key_hash = generate_api_key()
         api_key = APIKey(