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_search_scrape_ingest.py

@@ -104,7 +104,7 @@ def test_search_youtube_route_returns_422_when_ingest_fails(db, monkeypatch, ove
     from fastapi.testclient import TestClient
 
     import app.state as state
-    from app.auth import require_human
+    from app.auth import current_user
     from app.db import get_db
     from app.main import app
     from app.routes import search as search_mod
@@ -146,7 +146,7 @@ def test_search_youtube_route_returns_422_when_ingest_fails(db, monkeypatch, ove
     monkeypatch.setattr(search_mod, "_ingest_candidates", _boom)
 
     override_dep(get_db, db.info["request"])
-    override_dep(require_human, lambda: user)
+    override_dep(current_user, lambda: user)
     resp = TestClient(app).get("/api/search/youtube?q=test+term")
 
     assert resp.status_code == 422, resp.text