feat!: remove the shared demo account

The demo account — one shared user that whitelisted emails entered from
the login page, reset on a schedule — goes end to end (owner decision,
E34): demo.py and the reset job, users.is_demo and demo_whitelist
(migration 0098 deletes the demo user, its audit rows, the reset job's
override and notices, then drops the column and table; the downgrade
restores the empty schema), the /auth/demo entry, the admin whitelist and
reset, the Demo tab, the banner, the login page's "Try the demo", and
every is_demo branch in routes, userscope, scheduler and the UI.

require_human only ever refused the demo account, so its 87 uses become
current_user (owner-approved); require_write_scope keeps its own check.
The login page's "or" divider now shows only beside the Google button:
without the demo entry under it, an instance without Google would have
said "or" to nothing.

A contract test (removedFeatures) fails on the word anywhere in the
backend, frontend, e2e or scripts — seen red on a planted hit. Size
ceilings tightened for the modules that shrank; admin.py is back under
the limit.

frontend/src/components/Header.tsx

@@ -23,8 +23,7 @@ const Header = forwardRef<
   HTMLDivElement,
   {
     me: Me;
-    // Trigger a live YouTube search for the current term (feed only; hidden for the demo account,
-    // which can't spend the shared quota).
+    // Trigger a live YouTube search for the current term (feed only).
     onYtSearch: (q: string) => void;
   }
 >(function Header({ me, onYtSearch }, ref) {
@@ -64,7 +63,7 @@ const Header = forwardRef<
           me.search_source === "api" ? "header.searchYoutubeTipApi" : "header.searchYoutubeTip",
         )}
         goIcon={<YouTubeIcon className="w-4 h-4" />}
-        goDisabled={!trimmed || me.is_demo}
+        goDisabled={!trimmed}
       />
     );
   } else if (page === "plex" && me.plex_enabled) {