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_audit_count_rows.py

@@ -22,12 +22,6 @@ def test_clearing_the_audit_log_keeps_the_count_as_data(as_user, db, make_user):
     assert _after(db, "audit_clear") == {"cleared": 2}
 
 
-def test_a_demo_reset_keeps_the_count_as_data(as_user, db, make_user):
-    admin = make_user(email="boss@example.com", role="admin")
-    seeded = as_user(admin).post("/api/admin/demo/reset").json()["playlists_seeded"]
-    assert _after(db, "demo_reset") == {"playlists_seeded": seeded}
-
-
 def test_a_discovery_purge_keeps_its_counts_as_data(as_user, db, make_user):
     admin = make_user(email="boss@example.com", role="admin")
     body = as_user(admin).post("/api/admin/purge-discovery").json()