feat(auth): Complete OIDC security refactor and modernize test suite

- Refactored OIDC flow to implement PKCE, state/nonce validation, and BFF pattern.
- Centralized configuration in Settings class (DEV_MODE, FRONTEND_URL, OIDC_REDIRECT_URI).
- Updated auth routers to use conditional secure cookie flags based on DEV_MODE.
- Modernized and cleaned up test suite by removing legacy Streamlit tests.
- Fixed linting errors and unused imports across the backend.

frontend/src/services/auth.ts

@@ -28,11 +28,6 @@ export const AuthService = {
     }
   },
 
-  async exchangeOIDCCode(code: string): Promise<AuthResponse> {
-    const response = await api.get<AuthResponse>(`/auth/oidc/callback?code=${code}`)
-    return response.data
-  },
-
   async register(email: string, password: string): Promise<UserResponse> {
     const response = await api.post<UserResponse>("/auth/register", {
       email,