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/app.py

@@ -10,13 +10,15 @@
 # @DATA_CONTRACT [HTTP Request | WS Message] -> [HTTP Response | JSON Log Stream]
 # @RATIONALE Duplicate @RELATION and @INVARIANT lines removed from header. Import sorting unified via ruff isort (I) rule across src/ — 139 fixes applied.
 
+import asyncio
+from contextlib import asynccontextmanager
 import os
 from pathlib import Path
 
 # project_root is used for static files mounting
 project_root = Path(__file__).resolve().parent.parent.parent
-import asyncio
 
+from alembic.config import Config as AlembicConfig
 from fastapi import FastAPI, HTTPException, Request, WebSocket, WebSocketDisconnect
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import FileResponse
@@ -24,6 +26,8 @@ from fastapi.staticfiles import StaticFiles
 from starlette.middleware.base import BaseHTTPMiddleware
 from starlette.middleware.sessions import SessionMiddleware
 
+from alembic import command as alembic_command
+
 from .api import auth
 from .api.routes import (
     admin,
@@ -50,9 +54,6 @@ from .api.routes import (
     translate,
     validation,
 )
-from alembic.config import Config as AlembicConfig
-from alembic import command as alembic_command
-
 from .core.auth.security import get_password_hash
 from .core.cot_logger import seed_trace_id
 from .core.database import AuthSessionLocal, init_db
@@ -62,6 +63,28 @@ from .core.utils.network import NetworkError
 from .dependencies import get_scheduler_service, get_task_manager
 from .models.auth import Role, User
 
+
+# #region lifespan [C:3] [TYPE Function]
+# @BRIEF Async context manager for FastAPI startup/shutdown lifecycle.
+# @RELATION CALLS -> [run_alembic_migrations]
+# @RELATION CALLS -> [init_db]
+# @RELATION CALLS -> [AppDependencies]
+# @POST On startup: schema up-to-date, admin exists, scheduler started. On shutdown: scheduler stopped.
+@asynccontextmanager
+async def lifespan(app: FastAPI):
+    # Startup
+    seed_trace_id()
+    with belief_scope("startup_event"):
+        ensure_encryption_key()
+        run_alembic_migrations()
+        init_db()
+        ensure_initial_admin_user()
+        scheduler = get_scheduler_service()
+    scheduler.start()
+    yield
+    # Shutdown
+    scheduler.stop()
+# #endregion lifespan
 # #region FastAPI_App [C:3] [TYPE Global] [SEMANTICS app, fastapi, instance, route-registry]
 # @BRIEF Canonical FastAPI application instance for route, middleware, and websocket registration.
 # @RELATION DEPENDS_ON -> [ApiRoutesModule]
@@ -70,6 +93,7 @@ app = FastAPI(
     title="Superset Tools API",
     description="API for managing Superset automation tools and plugins.",
     version="1.0.0",
+    lifespan=lifespan,
 )
 # #endregion FastAPI_App
 # #region ensure_initial_admin_user [C:3] [TYPE Function]
@@ -152,37 +176,7 @@ def run_alembic_migrations() -> None:
 # #endregion run_alembic_migrations
 
 
-# #region startup_event [C:3] [TYPE Function]
-# @BRIEF Handles application startup: Alembic → create_all → admin bootstrap → scheduler.
-# @RELATION CALLS -> [run_alembic_migrations]
-# @RELATION CALLS -> [init_db]
-# @POST Schema is up-to-date via Alembic, missing tables created, admin exists, scheduler started.
-@app.on_event("startup")
-async def startup_event():
-    seed_trace_id()
-    with belief_scope("startup_event"):
-        ensure_encryption_key()
-        # 1. Alembic — applies ALL pending migrations (add/drop/rename columns, data migrations)
-        run_alembic_migrations()
-        # 2. init_db — creates new tables via create_all() (does NOT alter existing ones)
-        init_db()
-        ensure_initial_admin_user()
-        scheduler = get_scheduler_service()
-    scheduler.start()
-# #endregion startup_event
-# #region shutdown_event [C:3] [TYPE Function]
-# @BRIEF Handles application shutdown tasks, such as stopping the scheduler.
-# @RELATION CALLS -> [AppDependencies]
-# @PRE None.
-# @POST Scheduler is stopped.
-# Shutdown event
-@app.on_event("shutdown")
-async def shutdown_event():
-    seed_trace_id()
-    with belief_scope("shutdown_event"):
-        scheduler = get_scheduler_service()
-        scheduler.stop()
-# #endregion shutdown_event
+
 # #region app_middleware [TYPE Block]
 # @BRIEF Configure application-wide middleware (Session, CORS).
 # @RATIONALE SessionMiddleware uses SESSION_SECRET_KEY independent of JWT SECRET_KEY (see [SEC:H-4]).
@@ -349,8 +343,8 @@ async def _authenticate_websocket(websocket: WebSocket, endpoint_name: str) -> b
 
     # Try JWT first, fallback to API key
     try:
-        from .core.auth.jwt import decode_token
         from .core.auth.api_key import hash_api_key
+        from .core.auth.jwt import decode_token
         from .core.database import SessionLocal
 
         # Attempt JWT validation
@@ -479,7 +473,7 @@ async def websocket_endpoint(
             initial_sent = 0
             for log_entry in initial_logs:
                 if matches_filters(log_entry):
-                    log_dict = log_entry.dict()
+                    log_dict = log_entry.model_dump()
                     log_dict["timestamp"] = log_dict["timestamp"].isoformat()
                     await websocket.send_json(log_dict)
                     initial_sent += 1
@@ -510,7 +504,7 @@ async def websocket_endpoint(
                 log_entry = await queue.get()
                 if not matches_filters(log_entry):
                     continue
-                log_dict = log_entry.dict()
+                log_dict = log_entry.model_dump()
                 log_dict["timestamp"] = log_dict["timestamp"].isoformat()
                 await websocket.send_json(log_dict)
                 logger.reflect(