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_datasets.py

@@ -3,9 +3,9 @@
 # @SEMANTICS: tests, datasets, api, metrics, inline-edit
 # @PURPOSE: Contract tests for dataset endpoints (list+stats, detail+metrics, inline-edit descriptions).
 # @LAYER Tests
+import pytest
 from unittest.mock import AsyncMock, MagicMock, patch
 
-import pytest
 from fastapi.testclient import TestClient
 
 from src.app import app
@@ -396,7 +396,7 @@ def test_update_column_description_strips_html(mock_deps):
         assert response.status_code == 200
         data = response.json()
         # HTML stripped
-        assert "Hello World with alert('xss')" == data["description"]
+        assert data["description"] == "Hello World with alert('xss')"
         assert "<b>" not in data["description"]
         assert "<script>" not in data["description"]