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/admin.ts

@@ -1,18 +1,9 @@
 // The admin surface: discovery purge, system configuration, users & roles, the audit log, the
-// diagnostics console's server log, the scheduler + maintenance controls, the demo-account
-// whitelist, and access invites. Only touches core's req. (Grouped by sub-area below; kept in one
+// diagnostics console's server log, the scheduler + maintenance controls, and access invites. Only touches core's req. (Grouped by sub-area below; kept in one
 // module as the cohesive "admin" domain.)
 import type { Level } from "../log";
 import { req } from "./core";
 
-export interface DemoWhitelistEntry {
-  id: number;
-  email: string;
-  note: string | null;
-  added_by: string | null;
-  created_at: string | null;
-}
-
 export interface Invite {
   id: number;
   email: string;
@@ -78,7 +69,6 @@ export interface AdminUserRow {
   is_active: boolean;
   is_suspended: boolean;
   email_verified: boolean;
-  is_demo: boolean;
   has_password: boolean;
   /** When SEC-10 cleared this account's password, or null. Separates "never had one" (a Google-only
    *  account) from "we took it away" — different problems needing different help. */
@@ -87,7 +77,7 @@ export interface AdminUserRow {
   /** The Google account's address, ONLY when it differs from the account address (E9.5 S3). */
   google_email: string | null;
   created_at: string | null;
-  /** Last sign-in (Google or password). Null = not since 0.88 (no backfill), or the demo account. */
+  /** Last sign-in (Google or password). Null = not since 0.88 (no backfill). */
   last_login_at: string | null;
 }
 
@@ -288,13 +278,6 @@ export const adminApi = {
       method: "PATCH",
       body: JSON.stringify({ revalidate_batch: revalidateBatch }),
     }),
-  // --- demo whitelist ---
-  adminDemoWhitelist: (): Promise<DemoWhitelistEntry[]> => req("/api/admin/demo/whitelist"),
-  addDemoWhitelist: (email: string): Promise<DemoWhitelistEntry> =>
-    req("/api/admin/demo/whitelist", { method: "POST", body: JSON.stringify({ email }) }),
-  removeDemoWhitelist: (id: number) => req(`/api/admin/demo/whitelist/${id}`, { method: "DELETE" }),
-  resetDemo: (): Promise<{ reset: boolean; playlists_seeded: number }> =>
-    req("/api/admin/demo/reset", { method: "POST" }),
   // --- access invites ---
   // Sign-ups that registered but still cannot get in — the admin lifeline for roadmap UX-25.
   adminPendingAccess: (): Promise<PendingAccessRow[]> => req("/api/admin/pending-access"),