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_persistence.py

@@ -5,7 +5,7 @@
 # @LAYER Tests
 # @TEST_DATA: valid_task -> {"id": "test-uuid-1", "plugin_id": "backup", "status": "PENDING"}
 
-from datetime import datetime
+from datetime import UTC, datetime
 from unittest.mock import patch
 
 from sqlalchemy import create_engine
@@ -17,7 +17,6 @@ from src.models.mapping import Base, Environment
 from src.models.task import TaskRecord
 
 
-
 # #region TestTaskPersistenceHelpers [C:2] [TYPE Class]
 # @RELATION  BINDS_TO -> test_task_persistence
 # @PURPOSE: Test suite for TaskPersistenceService static helper methods.
@@ -194,7 +193,7 @@ class TestTaskPersistenceService:
 
         # Update status
         task.status = TaskStatus.RUNNING
-        task.started_at = datetime.utcnow()
+        task.started_at = datetime.now(UTC)
 
         with self._patched():
             self.service.persist_task(task)
@@ -283,8 +282,8 @@ class TestTaskPersistenceService:
         """Test loading tasks from database (round-trip)."""
         task = self._make_task(
             status=TaskStatus.SUCCESS,
-            started_at=datetime.utcnow(),
-            finished_at=datetime.utcnow(),
+            started_at=datetime.now(UTC),
+            finished_at=datetime.now(UTC),
         )
         task.params = {"key": "value"}
         task.result = {"output": "done"}