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/DebugRecorder.tsx

@@ -36,7 +36,7 @@ import { SURFACE } from "./ui/surfaces";
 // While a session records, its pill sits under the toasts (lib/toastDock), so it rides every toast
 // host — the page, each dialog, both players, the fullscreen player — and is reachable wherever the
 // bug is being reproduced, above a player's control band rather than on it. Mounted once, in the
-// app shell, for a signed-in account other than the demo.
+// app shell, for a signed-in account.
 
 /** The time since `since`, as the app's other clocks show it, re-rendered every second. */
 function useElapsed(since: string): string {