Fix review findings and remove Uncategorized from sidebar

Backend: cache Google access tokens (drop dead access_token_expires_at,
migration 0007), handle MeTube cleared/canceled events by URL, return
email from /auth/status only when authenticated, move Google base URLs
into settings, run container as non-root.

Frontend: include local feed filters in the query key, remove dead
Saved page and unused assets, drop stale CategoryNav props and classes,
send Content-Type only with a body, remove Uncategorized from sidebar.

frontend/src/api/client.ts

@@ -13,7 +13,7 @@ export function authErrorMessage(code: string): string {
 async function request<T>(path: string, options: RequestInit = {}): Promise<T> {
   const res = await fetch(path, {
     credentials: 'include',
-    headers: { 'Content-Type': 'application/json', ...(options.headers ?? {}) },
+    headers: options.body != null ? { 'Content-Type': 'application/json', ...(options.headers ?? {}) } : options.headers,
     ...options,
   })
   if (!res.ok) {