style: fix all ruff and eslint errors across codebase

Backend (ruff):
- Fix 106 errors: move imports to top of file (E402)
- Remove unused imports (F401)
- Add missing imports for undefined names (F821)
- Remove unused variables (F841)
- Fix test_models.py broken RefreshToken test
- Fix test_projects_api.py missing TestClient import

Frontend (eslint):
- Remove unused imports/variables across 10 files
- Fix explicit any types in client.ts and sessions.ts
- Clean up empty block statements in terminal.tsx

Quality gates: ruff (pass), eslint (pass), tsc --noEmit (pass),
pytest (98 passed, 4 pre-existing failures)

apps/web/src/api/client.ts

@@ -1,4 +1,4 @@
-import axios from "axios";
+import axios, { type AxiosRequestConfig } from "axios";
 
 const BASE_URL = import.meta.env.VITE_API_BASE_URL ?? "http://localhost:8000";
 
@@ -19,7 +19,7 @@ const MAX_RETRIES = 2;
 const RETRY_DELAY_MS = 1000;
 
 // Track retry count per request
-const retryCount = new WeakMap<any, number>();
+const retryCount = new WeakMap<AxiosRequestConfig, number>();
 
 apiClient.interceptors.response.use(
   (response) => response,