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/bodylimit.py

@@ -61,7 +61,7 @@ DEFAULT_MAX_BODY_BYTES = 1024 * 1024
 # Both listed routes now satisfy that. The screenshot route always did (`capped_body` streams it,
 # and the owner check resolves first). The upload route did NOT until P1: it declared
 # `file: UploadFile = File(...)`, so an anonymous caller could make the process write the whole
-# per-file allowance to temp before `require_human` refused them — this middleware bounded that
+# per-file allowance to temp before the auth dependency refused them — this middleware bounded that
 # number but could not stop the writing. It now reads its own body from a dependency declared after
 # the auth one, so its in-handler cap measures bytes as they arrive, like everything else here.
 #