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/src/services/reports/normalizer.py

@@ -10,7 +10,7 @@
 # @POST  Returns Normalizer output with normalized fields
 # @SIDE_EFFECT  Read-only database operations
 
-from datetime import datetime
+from datetime import UTC, datetime
 from typing import Any
 
 from ...core.logger import belief_scope
@@ -126,7 +126,7 @@ def normalize_task_report(task: Task) -> TaskReport:
         started_at = task.started_at if isinstance(task.started_at, datetime) else None
         updated_at = task.finished_at if isinstance(task.finished_at, datetime) else None
         if not updated_at:
-            updated_at = started_at or datetime.utcnow()
+            updated_at = started_at or datetime.now(UTC)
 
         details: dict[str, Any] = {
             "profile": {