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/App.tsx

@@ -42,7 +42,6 @@ import AppCommandPalette from "./components/AppCommandPalette";
 import { useGlobalKeymap } from "./lib/keymap";
 import ErrorDialog from "./components/ErrorDialog";
 import VersionBanner from "./components/VersionBanner";
-import DemoBanner from "./components/DemoBanner";
 import VerifyBanner from "./components/VerifyBanner";
 import { focusAccessRequestsTab } from "./lib/adminUsersTab";
 // The built app version, NOT imported from releaseNotes — importing anything from that module
@@ -438,7 +437,6 @@ export default function App() {
             banners={
               <>
                 <VerifyBanner entry={verifyEntry} />
-                {me!.is_demo && <DemoBanner />}
                 <VersionBanner onOpen={() => openReleaseNotes(FRONTEND_VERSION)} />
               </>
             }
@@ -459,7 +457,7 @@ export default function App() {
       <KeyboardHelp />
       <AppCommandPalette />
       {/* The debug recorder (E16): its start dialog, and its pill docked under the toasts. */}
-      {me && !me.is_demo && <DebugRecorder />}
+      {me && <DebugRecorder />}
       <Suspense fallback={null}>
         {aboutOpen && (
           <About