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/models/git.py

@@ -1,6 +1,6 @@
 # #region GitModels [TYPE Module] [SEMANTICS sqlalchemy, git, model, config, sync]
 
-from datetime import datetime
+from datetime import UTC, datetime
 import enum
 import uuid
 
@@ -37,7 +37,7 @@ class GitServerConfig(Base):
     default_repository = Column(String(255), nullable=True)
     default_branch = Column(String(255), default="main")
     status = Column(Enum(GitStatus), default=GitStatus.UNKNOWN)
-    last_validated = Column(DateTime, default=datetime.utcnow)
+    last_validated = Column(DateTime, default=lambda: datetime.now(UTC))
 # #endregion GitServerConfig
 
 # #region GitRepository [C:1] [TYPE Class]