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/tests/test_userscope.py

@@ -1,16 +1,16 @@
-"""is_messageable_user — the "real, active, non-suspended human" predicate reused by the messaging
-directory and the download share endpoints. A row must be non-demo AND active AND not-suspended."""
+"""is_messageable_user — the "active, non-suspended account" predicate reused by the download share
+endpoints. A row must be active AND not-suspended."""
 
 from types import SimpleNamespace
 
 from app.userscope import is_messageable_user
 
 
-def _user(is_demo=False, is_active=True, is_suspended=False):
-    return SimpleNamespace(is_demo=is_demo, is_active=is_active, is_suspended=is_suspended)
+def _user(is_active=True, is_suspended=False):
+    return SimpleNamespace(is_active=is_active, is_suspended=is_suspended)
 
 
-def test_a_real_active_human_is_messageable():
+def test_an_active_account_is_messageable():
     assert is_messageable_user(_user()) is True
 
 
@@ -18,7 +18,6 @@ def test_none_is_not_messageable():
     assert is_messageable_user(None) is False
 
 
-def test_demo_suspended_or_inactive_are_not_messageable():
-    assert is_messageable_user(_user(is_demo=True)) is False
+def test_suspended_or_inactive_are_not_messageable():
     assert is_messageable_user(_user(is_suspended=True)) is False
     assert is_messageable_user(_user(is_active=False)) is False