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/lib/api/core.test.ts

@@ -215,7 +215,6 @@ describe("req — a structured refusal reads in the user's language", () => {
     [{ code: "activate", reason: "not_allowed" }, "errors.admin.not_allowed"],
     [{ code: "invite_send", reason: "not_approved" }, "errors.admin.not_approved"],
     [{ code: "resend", reason: "send_failed" }, "errors.admin.send_failed"],
-    [{ code: "setpw", reason: "demo" }, "errors.admin.demo"],
   ])("says why an admin action was refused (%o)", async (detail, localized) => {
     // These read as the generic "try again" — which no retry fixes when the address is not approved.
     vi.spyOn(globalThis, "fetch").mockResolvedValue(