feat(auth): refresh tokens + remember me

Backend: add refresh JWT (30d, token_type claim), POST /auth/refresh
endpoint (rotates token pair), remember_me on login, JWT_REFRESH_EXPIRY_DAYS
env var. Extractors now reject refresh tokens on protected routes.

Frontend: sessionStorage for non-remembered sessions, localStorage +
refresh token for remembered sessions. Transparent 401 recovery in
api.ts (retry once after refresh). Remember me checkbox on login page
with security note when checked.

k-tv-frontend/lib/types.ts

@@ -178,6 +178,7 @@ export interface TokenResponse {
   access_token: string;
   token_type: string;
   expires_in: number;
+  refresh_token?: string;
 }
 
 export interface UserResponse {