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/i18n/locales/en/settings.json

@@ -73,7 +73,6 @@
     "deleteTitle": "Delete your account?",
     "deleteConfirm": "This permanently erases your account and all your data (subscriptions, tags, watch/saved/hidden state, playlists, settings). It can't be undone.",
     "deleteConfirmButton": "Delete everything",
-    "demoNotice": "This is the shared demo account. It has no YouTube connection — you can browse the whole shared Library, filter and sort it, build playlists and try things out, but nothing touches a real YouTube account.",
     "signInMethods": "Sign-in methods",
     "googleLink": {
       "title": "Google account",
@@ -132,28 +131,6 @@
     "removeConfirmAction": "Delete",
     "loadError": "Couldn't load your download authentication settings"
   },
-  "demo": {
-    "title": "Demo access",
-    "intro": "Emails here can enter the shared demo account straight from the login page — no Google sign-in. They just type the address into the field and are let in after a moment.",
-    "addPlaceholder": "Whitelist an email for demo…",
-    "add": "Add",
-    "added": "{{email}} added to the demo whitelist",
-    "addFailed": "Couldn't add that email",
-    "removeFailed": "Couldn't remove that email",
-    "remove": "Remove",
-    "removeHint": "Remove this email from the demo whitelist.",
-    "addedBy": "added by {{who}}",
-    "reset": "Reset demo state",
-    "resetHint": "Wipe the demo account's playlists, hidden/watched/saved videos and settings, then re-seed a few sample playlists.",
-    "resetTitle": "Reset demo state?",
-    "resetConfirm": "This clears everything the demo account has accumulated — playlists, hidden/watched/saved videos and settings — and re-seeds a few sample playlists. It can't be undone.",
-    "resetDone_one": "Demo reset — {{count}} sample playlist seeded",
-    "resetDone_other": "Demo reset — {{count}} sample playlists seeded",
-    "resetFailed": "Couldn't reset the demo account",
-    "removed": "Removed from the demo whitelist",
-    "removeTitle": "Remove demo access?",
-    "removeConfirm": "Remove {{email}} from the demo whitelist? They'll no longer be able to enter the demo account."
-  },
   "invites": {
     "title": "Access requests",
     "noPending": "No pending requests.",
@@ -228,7 +205,6 @@
     },
     "status": "{{level}} logging · back to “Basic” in {{time}}",
     "revert": "Back to “Basic” now",
-    "demo": "The demo account is shared by several people, so detailed logging can't be turned on here.",
     "failed": "Couldn't change the logging level"
   },
   "myLog": {