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/accountPanel.dom.test.tsx

@@ -58,7 +58,6 @@ const user = {
   email: "stuck@example.com",
   display_name: null,
   role: "user",
-  is_demo: false,
   is_active: false,
   is_suspended: false,
 };
@@ -148,15 +147,3 @@ describe("the account panel reports a refused recovery action", () => {
     await waitFor(() => expect(errorMessages()).toContain("users.pendingAccess.activateFailed"));
   });
 });
-
-describe("the account panel on the demo account", () => {
-  // The demo account's address is never confirmed, so "send the confirmation link again" looked
-  // available — and the server refused it as "nothing left to confirm", beside a row reading
-  // "unconfirmed" (deep-review E11 round 3).
-  it("does not offer it a confirmation mail, which the server refuses", async () => {
-    mount({ ...user, is_demo: true });
-    await screen.findByText("users.account.recovery");
-    const resend = screen.getByText("users.account.resend").closest("button")!;
-    expect(resend.hasAttribute("disabled")).toBe(true);
-  });
-});