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/gates.snapshot.json

@@ -1499,6 +1499,7 @@
         "src/lib/reactEngine.contract.test.ts",
         "src/lib/reinsert.test.ts",
         "src/lib/releaseNotes.test.ts",
+        "src/lib/removedFeatures.contract.test.ts",
         "src/lib/repoScripts.contract.test.ts",
         "src/lib/rovingHighlight.contract.test.ts",
         "src/lib/saveLanguage.dom.test.ts",
@@ -1565,7 +1566,6 @@
         "src/components/ConfirmProvider.tsx",
         "src/components/DateFilterSelect.tsx",
         "src/components/DebugRecorder.tsx",
-        "src/components/DemoBanner.tsx",
         "src/components/DiagnosticsSettings.tsx",
         "src/components/DownloadButton.tsx",
         "src/components/DownloadCenter.tsx",