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/app/routes/search.py

@@ -5,8 +5,8 @@ with the normal feed cards + in-app player and gain per-user state (watch / save
 source is admin-selectable (``search_source``): **scrape** (InnerTube, the website's own
 endpoint, zero API quota — the default) or **api** (search.list, **100 quota units per page**).
 Either way it's gated behind a per-user daily cap, the API source additionally behind a budget
-pre-check, and only real users (never the shared demo account) may call it. The result ids are
-enriched through the cheap shared videos.list path (1 unit / 50) regardless of source.
+pre-check. The result ids are enriched through the cheap shared videos.list path (1 unit / 50)
+regardless of source.
 
 Ingested results are flagged (videos.via_search / channels.from_search) so the Library can hide
 this search-discovered "noise" by default. Shorts and currently-live/upcoming videos are never
@@ -23,7 +23,7 @@ from sqlalchemy.dialects.postgresql import insert as pg_insert
 from sqlalchemy.orm import Session, aliased
 
 from app import logs, quota, sysconfig
-from app.auth import require_human
+from app.auth import current_user
 from app.db import get_db
 from app.models import (
     LIVE_OR_UPCOMING,
@@ -251,7 +251,7 @@ def search_youtube(
     q: str | None = None,
     cursor: str | None = None,
     limit: int = 20,
-    user: User = Depends(require_human),
+    user: User = Depends(current_user),
     db: Session = Depends(get_db),
 ) -> dict:
     """Live YouTube search results materialised into the catalog. `limit` is how many results to
@@ -402,7 +402,7 @@ class ClearSearchIn(BaseModel):
 @router.post("/clear")
 def clear_search(
     body: ClearSearchIn,
-    user: User = Depends(require_human),
+    user: User = Depends(current_user),
     db: Session = Depends(get_db),
 ) -> dict:
     """Discard the given live-search results "as if never added": drop this user's search-finds