chore(lint): apply ruff --fix (4443 auto-fixes)

Auto-fixed categories:
- F401: unused imports removed
- I001: import blocks sorted
- W293: trailing whitespace stripped
- UP035: deprecated typing imports replaced
- SIM: simplify suggestions applied
- ARG: unused args prefixed with underscore
- T201: print statements removed
- F841: unused variables removed
- RUF059: unpacked variables prefixed

Remaining ~1500 unfixable errors (C901, B904, N806, E402) require manual work.

Backend smoke tests: 13/13 passed.

backend/src/app.py

@@ -11,46 +11,50 @@
 # @DATA_CONTRACT: [HTTP Request | WS Message] -> [HTTP Response | JSON Log Stream]
 import os
 from pathlib import Path
+
 # project_root is used for static files mounting
 project_root = Path(__file__).resolve().parent.parent.parent
-from fastapi import FastAPI, WebSocket, WebSocketDisconnect, Request, HTTPException
-from starlette.middleware.sessions import SessionMiddleware
-from fastapi.middleware.cors import CORSMiddleware
-from fastapi.staticfiles import StaticFiles
-from fastapi.responses import FileResponse
 import asyncio
-from .dependencies import get_task_manager, get_scheduler_service
-from .core.encryption_key import ensure_encryption_key
-from .core.utils.network import NetworkError
-from .core.logger import logger, belief_scope
-from .core.cot_logger import seed_trace_id
-from .core.database import AuthSessionLocal
-from .core.auth.security import get_password_hash
-from .models.auth import User, Role
+
+from fastapi import FastAPI, HTTPException, Request, WebSocket, WebSocketDisconnect
+from fastapi.middleware.cors import CORSMiddleware
+from fastapi.responses import FileResponse
+from fastapi.staticfiles import StaticFiles
+from starlette.middleware.sessions import SessionMiddleware
+
+from .api import auth
 from .api.routes import (
-    plugins,
-    tasks,
-    settings,
-    environments,
-    mappings,
-    migration,
-    connections,
-    git,
-    storage,
     admin,
-    llm,
-    dashboards,
-    datasets,
-    reports,
     assistant,
     clean_release,
     clean_release_v2,
-    profile,
-    health,
+    connections,
+    dashboards,
     dataset_review,
+    datasets,
+    environments,
+    git,
+    health,
+    llm,
+    mappings,
+    migration,
+    plugins,
+    profile,
+    reports,
+    settings,
+    storage,
+    tasks,
     translate,
 )
-from .api import auth
+from .core.auth.security import get_password_hash
+from .core.cot_logger import seed_trace_id
+from .core.database import AuthSessionLocal
+from .core.encryption_key import ensure_encryption_key
+from .core.logger import belief_scope, logger
+from .core.utils.network import NetworkError
+from .dependencies import get_scheduler_service, get_task_manager
+from .models.auth import Role, User
+
 # #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]
@@ -141,6 +145,7 @@ async def shutdown_event():
 # @BRIEF Configure application-wide middleware (Session, CORS).
 # Configure Session Middleware (required by Authlib for OAuth2 flow)
 from .core.auth.config import auth_config
+
 app.add_middleware(SessionMiddleware, secret_key=auth_config.SECRET_KEY)
 # Configure CORS
 app.add_middleware(
@@ -193,6 +198,7 @@ async def log_requests(request: Request, call_next):
 # Seed trace_id on every request — MUST be the outermost middleware so trace_id
 # is seeded before log_requests (or any other middleware) runs.
 from .core.middleware.trace import TraceContextMiddleware
+
 app.add_middleware(TraceContextMiddleware)
 # #region API_Routes [C:3] [TYPE Block]
 # @BRIEF Register all FastAPI route groups exposed by the application entrypoint.
@@ -425,4 +431,4 @@ else:
             }
     # #endregion read_root
 # #endregion StaticFiles
-# #endregion AppModule
+# #endregion AppModule