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/core/task_manager/lifecycle.py

@@ -127,7 +127,7 @@ class JobLifecycle:
                 f"Starting execution of task {task_id} for plugin '{plugin.name}'"
             )
             task.status = TaskStatus.RUNNING
-            task.started_at = datetime.utcnow()
+            task.started_at = datetime.now(UTC)
             self.persistence_service.persist_task(task)
             if add_log_callback:
                 add_log_callback(
@@ -181,7 +181,7 @@ class JobLifecycle:
                         metadata={"error_type": type(e).__name__},
                     )
             finally:
-                task.finished_at = datetime.utcnow()
+                task.finished_at = datetime.now(UTC)
                 self.event_bus.flush_task_logs(task_id)
                 self.persistence_service.persist_task(task)
                 logger.info(