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_task_manager.py

@@ -5,16 +5,15 @@
 # @LAYER Core
 # @INVARIANT: TaskManager state changes are deterministic and testable with mocked dependencies.
 
-import sys
 from pathlib import Path
+import sys
 
 sys.path.insert(0, str(Path(__file__).parent.parent / "src"))
 
 import asyncio
 from datetime import datetime
-from unittest.mock import MagicMock, patch
-
 import pytest
+from unittest.mock import MagicMock, patch
 
 
 # Helper to create a TaskManager with mocked dependencies