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/modules.ts

@@ -41,13 +41,12 @@ export type ModuleDef = {
   // which owns the polling queries; this only flags which modules have one).
   hasBadge?: boolean;
   // Availability for THIS account, derived from language-independent account state (plex toggle,
-  // demo restrictions, admin role). Absent = always available. Drives both the reachable module
+  // admin role). Absent = always available. Drives both the reachable module
   // set (nav + stepper) and, later, the render gate.
   gate?: (me: Me) => boolean;
 };
 
 const isAdmin = (me: Me) => me.role === "admin";
-const notDemo = (me: Me) => !me.is_demo;
 
 // Ordered in nav-rail order — this array IS the sequence the rail and the stepper walk.
 const MODULES: readonly ModuleDef[] = [
@@ -95,7 +94,6 @@ const MODULES: readonly ModuleDef[] = [
     railKey: "nav.rail.downloads",
     icon: Download,
     hasBadge: true,
-    gate: notDemo,
   },
   {
     id: "stats",