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_set_password_link.py

@@ -155,18 +155,6 @@ def test_only_an_admin_can_mint_one(db, as_user, make_user):
     assert db.query(AuthToken).filter(AuthToken.kind == "setpw").count() == 0
 
 
-def test_the_shared_demo_account_is_refused(db, as_user, make_user):
-    """The demo account is shared, so handing anyone a way to change its password hands them
-    everyone else's session on it."""
-    admin = make_user(role="admin")
-    demo = make_user(is_demo=True)
-    db.commit()
-
-    r = as_user(admin).post(f"/api/admin/users/{demo.id}/set-password-link")
-    assert r.status_code == 400
-    assert r.json()["detail"]["reason"] == "demo"
-
-
 def test_the_access_panel_reports_the_gates(db, as_user, make_user):
     admin = make_user(role="admin")
     target = make_user(email_verified=False, is_active=False)