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_debug_sessions.py

@@ -2,7 +2,7 @@
 
 Four parts:
   * the session's life: one open per user, a second Start joins it, a Stop ends it, the cap ends it
-    on its own, the demo account cannot record, nobody reaches another user's session (DB lane);
+    on its own, nobody reaches another user's session (DB lane);
   * the tag: a request naming the caller's own open session is logged at its level with every line
     tagged, while a header naming anything else is ignored (a real sign-in through `_session_user`);
   * the browser's batches: a session counts as consent to its level, and only its own events keep
@@ -71,11 +71,6 @@ def test_only_the_two_raised_levels_can_be_recorded(db, make_user, as_user):
         assert client.post("/api/debug-sessions", json={"level": bogus}).status_code == 422
 
 
-def test_the_demo_account_cannot_record(db, make_user, as_user):
-    r = as_user(make_user(is_demo=True)).post("/api/debug-sessions", json={"level": "DEBUG"})
-    assert r.status_code == 403 and r.json()["detail"]["code"] == "demo"
-
-
 def test_stop_ends_it_and_me_stops_reporting_it(db, client, make_user):
     _signed_in(client, make_user)
     sid = _start(client)["id"]