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_api_key_model.py

@@ -6,7 +6,6 @@
 # @TEST_EDGE: prefix is exactly 11 chars ("ssk_" + 7)
 # @TEST_EDGE: active defaults to True
 import pytest
-from datetime import datetime, timezone
 
 
 # #region clean_db [C:1] [TYPE Fixture]
@@ -16,6 +15,7 @@ def clean_db():
     from sqlalchemy import create_engine
     from sqlalchemy.orm import sessionmaker
     from sqlalchemy.pool import StaticPool
+
     from src.models.mapping import Base
 
     engine = create_engine(
@@ -65,6 +65,7 @@ class TestAPIKeyModel:
     # @BRIEF key_hash is unique — duplicate raises IntegrityError.
     def test_key_hash_unique(self, clean_db):
         from sqlalchemy.exc import IntegrityError
+
         from src.models.api_key import APIKey
 
         clean_db.add(APIKey(