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/account.ts

@@ -9,7 +9,6 @@ export interface Me {
   display_name: string | null;
   avatar_url: string | null;
   role: string;
-  is_demo: boolean;
   /** The latest sign-in on any device (ISO), or null when none is recorded. */
   last_login_at: string | null;
   has_google: boolean;
@@ -40,10 +39,9 @@ export interface Me {
   issues: { repo: string } | null;
 }
 
-/** The `nav.role.*` key an account is shown under. Demo isn't a real role (it's a flag on a `user`
- *  row), so it comes first. */
-export function roleKey(a: Pick<Me, "is_demo" | "role">): "admin" | "user" | "demo" {
-  return a.is_demo ? "demo" : a.role === "admin" ? "admin" : "user";
+/** The `nav.role.*` key an account is shown under. */
+export function roleKey(a: Pick<Me, "role">): "admin" | "user" {
+  return a.role === "admin" ? "admin" : "user";
 }
 
 /** What an account is called on screen: its display name, else the local part of its email. */