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/SettingsPanel.account.dom.test.tsx

@@ -33,7 +33,6 @@ const me = (extra: Partial<Me>): Me =>
     display_name: "Someone",
     avatar_url: null,
     role: "user",
-    is_demo: false,
     last_login_at: null,
     ...extra,
   }) as Me;
@@ -47,14 +46,10 @@ describe("AccountHeader", () => {
     expect(screen.getByText("nav.role.user")).toBeTruthy();
   });
 
-  it("says when none is recorded, and says nothing about it for the demo", () => {
+  it("says when none is recorded", () => {
     render(<AccountHeader me={me({})} />);
     expect(screen.getByTestId("account-last-login").textContent).toBe(
       "settings.account.lastLoginNone",
     );
-    cleanup();
-    render(<AccountHeader me={me({ is_demo: true })} />);
-    expect(screen.queryByTestId("account-last-login")).toBeNull();
-    expect(screen.getByText("nav.role.demo")).toBeTruthy();
   });
 });