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/feed.json

@@ -17,9 +17,6 @@
   "emptyBody": "Connect your YouTube account to import your subscriptions and build your Feed.",
   "setUp": "Set up my Feed",
   "browseShared": "Or just browse the shared Library",
-  "demoEmptyTitle": "Nothing here in the demo",
-  "demoEmptyBody": "The demo account has no subscriptions of its own — but the whole shared Library is yours to explore, filter and sort.",
-  "browseLibrary": "Browse the Library",
   "noMatches": "No videos match these filters.",
   "channelScopeHint": "This channel isn't in your Feed — it may be hidden. Its videos are in the shared Library.",
   "viewInLibrary": "View in Library",