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.searchTip.dom.test.tsx

@@ -31,7 +31,7 @@ import type { Me } from "../lib/api";
 afterEach(cleanup);
 
 function tipFor(source: Me["search_source"]): string | null {
-  const me = { is_demo: false, plex_enabled: false, search_source: source } as Me;
+  const me = { plex_enabled: false, search_source: source } as Me;
   render(<Header me={me} onYtSearch={() => {}} />);
   return screen.getByRole("button", { name: /header\.searchYoutube$/ }).getAttribute("title");
 }