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.

backend/app/README.md

@@ -1,7 +1,7 @@
 # Backend conventions
 
 The rules below are already followed across `backend/app` — they lived only in a few module
-docstrings (`demo.py`, `userscope.py`, `routes/_common.py`) until this file collected them (E3 S3,
+docstrings (`userscope.py`, `routes/_common.py`) until this file collected them (E3 S3,
 B-core-20). When you add code, match these so the next reader doesn't have to guess "which pattern do
 I copy".