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_maintenance_api.py

@@ -10,20 +10,13 @@
 # @TEST_EDGE: missing_tables -> 422 validation error
 # @TEST_EDGE: end_time_before_start_time -> 400 validation error
 # @TEST_EDGE: non_existent_event_end -> 404 not found
-import json
-import os
-import uuid
-from datetime import datetime, timezone, timedelta
-from typing import Any
-from unittest.mock import AsyncMock, MagicMock, patch, PropertyMock
-
+from datetime import UTC, datetime, timedelta
 import pytest
-from fastapi import status
 
 # Patch GitService at module level to prevent /app/storage/repositories error
 # This patches the base module BEFORE any imports can reach it
 import sys
-from unittest.mock import MagicMock
+from unittest.mock import AsyncMock, MagicMock
 
 _mock_git_svc_cls = MagicMock()
 _mock_git_svc_cls.return_value = MagicMock()
@@ -37,6 +30,7 @@ sys.modules['src.services.git_service'] = MagicMock(GitService=_mock_git_svc_cls
 def client():
     """Create a TestClient with all dependencies mocked."""
     from fastapi.testclient import TestClient
+
     from src.app import app
     return TestClient(app)
 
@@ -44,10 +38,10 @@ def client():
 @pytest.fixture(autouse=True)
 def auth_mock(client, mock_db):
     """Create a valid API key with maintenance permissions and mock JWT user."""
-    from src.core.auth.api_key import generate_api_key
-    from src.models.api_key import APIKey
-    from src.dependencies import get_current_user
     from src.app import app
+    from src.core.auth.api_key import generate_api_key
+    from src.dependencies import get_current_user
+    from src.models.api_key import APIKey
 
     # Create API key for mutation endpoints
     raw, prefix, key_hash = generate_api_key()
@@ -93,11 +87,12 @@ def mock_db():
     Creates all tables on the shared Base.metadata.
     """
     from sqlalchemy import create_engine
-    from sqlalchemy.orm import sessionmaker, Session as SASession
+    from sqlalchemy.orm import sessionmaker
     from sqlalchemy.pool import StaticPool
-    from src.models.mapping import Base
+
     from src.dependencies import get_db
-    from src.models.maintenance import MaintenanceSettings, DashboardScope
+    from src.models.maintenance import DashboardScope, MaintenanceSettings
+    from src.models.mapping import Base
 
     # StaticPool ensures all sessions share the same in-memory connection
     engine = create_engine(
@@ -165,8 +160,8 @@ class TestStartEndpoint:
             "/api/maintenance/start",
             json={
                 "tables": ["raw.sales"],
-                "start_time": (datetime.now(timezone.utc) + timedelta(hours=1)).isoformat(),
-                "end_time": (datetime.now(timezone.utc) + timedelta(hours=3)).isoformat(),
+                "start_time": (datetime.now(UTC) + timedelta(hours=1)).isoformat(),
+                "end_time": (datetime.now(UTC) + timedelta(hours=3)).isoformat(),
                 "message": "Test maintenance",
                 "environment_id": "test-env",
             },
@@ -184,7 +179,7 @@ class TestStartEndpoint:
         response = client.post(
             "/api/maintenance/start",
             json={
-                "start_time": (datetime.now(timezone.utc) + timedelta(hours=1)).isoformat(),
+                "start_time": (datetime.now(UTC) + timedelta(hours=1)).isoformat(),
             },
         )
         assert response.status_code == 422
@@ -197,8 +192,8 @@ class TestStartEndpoint:
             "/api/maintenance/start",
             json={
                 "tables": ["raw.sales"],
-                "start_time": (datetime.now(timezone.utc) + timedelta(hours=3)).isoformat(),
-                "end_time": (datetime.now(timezone.utc) + timedelta(hours=1)).isoformat(),
+                "start_time": (datetime.now(UTC) + timedelta(hours=3)).isoformat(),
+                "end_time": (datetime.now(UTC) + timedelta(hours=1)).isoformat(),
                 "environment_id": "test-env",
             },
         )
@@ -213,7 +208,7 @@ class TestStartEndpoint:
             "/api/maintenance/start",
             json={
                 "tables": tables,
-                "start_time": (datetime.now(timezone.utc) + timedelta(hours=1)).isoformat(),
+                "start_time": (datetime.now(UTC) + timedelta(hours=1)).isoformat(),
                 "environment_id": "test-env",
             },
         )
@@ -227,8 +222,8 @@ class TestStartEndpoint:
 
         payload = {
             "tables": ["raw.sales"],
-            "start_time": (datetime.now(timezone.utc) + timedelta(hours=1)).isoformat(),
-            "end_time": (datetime.now(timezone.utc) + timedelta(hours=3)).isoformat(),
+            "start_time": (datetime.now(UTC) + timedelta(hours=1)).isoformat(),
+            "end_time": (datetime.now(UTC) + timedelta(hours=3)).isoformat(),
             "environment_id": "test-env",
         }
 
@@ -266,7 +261,7 @@ class TestEndEndpoint:
 
         event = MaintenanceEvent(
             tables=["raw.sales"],
-            start_time=datetime.now(timezone.utc),
+            start_time=datetime.now(UTC),
             status=MaintenanceEventStatus.ACTIVE,
             environment_id="test-env",
         )
@@ -315,7 +310,7 @@ class TestEventsEndpoint:
 
         active = MaintenanceEvent(
             tables=["raw.active"],
-            start_time=datetime.now(timezone.utc),
+            start_time=datetime.now(UTC),
             status=MaintenanceEventStatus.ACTIVE,
             environment_id="test-env",
         )
@@ -323,7 +318,7 @@ class TestEventsEndpoint:
 
         completed = MaintenanceEvent(
             tables=["raw.done"],
-            start_time=datetime.now(timezone.utc),
+            start_time=datetime.now(UTC),
             status=MaintenanceEventStatus.COMPLETED,
             environment_id="test-env",
         )
@@ -340,13 +335,14 @@ class TestEventsEndpoint:
     # #region test_expired_event_auto_ends [C:2] [TYPE Function]
     # @BRIEF Event past end_time with ACTIVE status → create_task called with auto-end params.
     def test_expired_event_auto_ends(self, client, mock_db, mock_task_manager):
-        from src.models.maintenance import MaintenanceEvent, MaintenanceEventStatus
         from datetime import timedelta
 
+        from src.models.maintenance import MaintenanceEvent, MaintenanceEventStatus
+
         event = MaintenanceEvent(
             tables=["raw.sales"],
-            start_time=datetime.now(timezone.utc) - timedelta(hours=2),
-            end_time=datetime.now(timezone.utc) - timedelta(hours=1),
+            start_time=datetime.now(UTC) - timedelta(hours=2),
+            end_time=datetime.now(UTC) - timedelta(hours=1),
             message="Expired maintenance",
             status=MaintenanceEventStatus.ACTIVE,
             environment_id="test-env",
@@ -371,13 +367,14 @@ class TestEventsEndpoint:
     # #region test_future_end_time_skipped [C:2] [TYPE Function]
     # @BRIEF Event with future end_time → no task created, skipped by < now filter.
     def test_future_end_time_skipped(self, client, mock_db, mock_task_manager):
-        from src.models.maintenance import MaintenanceEvent, MaintenanceEventStatus
         from datetime import timedelta
 
+        from src.models.maintenance import MaintenanceEvent, MaintenanceEventStatus
+
         event = MaintenanceEvent(
             tables=["raw.sales"],
-            start_time=datetime.now(timezone.utc) + timedelta(hours=1),
-            end_time=datetime.now(timezone.utc) + timedelta(hours=3),
+            start_time=datetime.now(UTC) + timedelta(hours=1),
+            end_time=datetime.now(UTC) + timedelta(hours=3),
             message="Future maintenance",
             status=MaintenanceEventStatus.ACTIVE,
             environment_id="test-env",
@@ -394,12 +391,13 @@ class TestEventsEndpoint:
     # #region test_no_end_time_skipped [C:2] [TYPE Function]
     # @BRIEF Event with end_time=None → no task created, skipped by isnot(None) filter.
     def test_no_end_time_skipped(self, client, mock_db, mock_task_manager):
-        from src.models.maintenance import MaintenanceEvent, MaintenanceEventStatus
         from datetime import timedelta
 
+        from src.models.maintenance import MaintenanceEvent, MaintenanceEventStatus
+
         event = MaintenanceEvent(
             tables=["raw.sales"],
-            start_time=datetime.now(timezone.utc) - timedelta(hours=2),
+            start_time=datetime.now(UTC) - timedelta(hours=2),
             end_time=None,
             message="No end time",
             status=MaintenanceEventStatus.ACTIVE,