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/VideoMenu.dom.test.tsx

@@ -9,7 +9,6 @@ import { FloatingTree } from "@floating-ui/react";
 import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
 
 import type { Video } from "../lib/api";
-import { qk } from "../lib/queryKeys";
 
 vi.mock("react-i18next", async (importOriginal) => {
   const actual = await importOriginal<typeof import("react-i18next")>();
@@ -99,7 +98,7 @@ function Feed({
   );
 }
 
-function setup(v: Video, { demo = false } = {}) {
+function setup(v: Video) {
   const handlers = {
     onState: vi.fn(),
     onToggleSave: vi.fn(),
@@ -107,7 +106,6 @@ function setup(v: Video, { demo = false } = {}) {
     onOpen: vi.fn(),
   };
   const qc = new QueryClient();
-  qc.setQueryData(qk.me(), { is_demo: demo });
   render(
     <QueryClientProvider client={qc}>
       <FloatingTree>
@@ -175,12 +173,6 @@ describe("a video's menu", () => {
     expect(await screen.findByText("playlists.noneYet")).toBeTruthy();
   });
 
-  it("has no download row on the demo account", () => {
-    setup(video({}), { demo: true });
-    rightClick();
-    expect(labels()).not.toContain("downloads.menu.download");
-  });
-
   it("the '…' opens the same menu, says so, and a second press closes it", () => {
     setup(video({}));
     expect(more().getAttribute("aria-haspopup")).toBe("menu");