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/sync.py

@@ -4,7 +4,7 @@ from sqlalchemy.orm import Session
 
 from app import audit, quota, state
 from app.audit import AuditAction
-from app.auth import admin_user, current_user, has_read_scope, require_human
+from app.auth import admin_user, current_user, has_read_scope
 from app.db import get_db
 from app.models import Channel, Subscription, User, Video
 from app.routes._common import youtube_refusal
@@ -26,7 +26,7 @@ _sync_limiter = RateLimiter(max_events=3, window_seconds=600)
 
 
 @router.post("/subscriptions")
-def sync_subscriptions(user: User = Depends(require_human), db: Session = Depends(get_db)) -> dict:
+def sync_subscriptions(user: User = Depends(current_user), db: Session = Depends(get_db)) -> dict:
     if not has_read_scope(user):
         # No YouTube read grant yet — the onboarding wizard hasn't been completed.
         raise refused(403, "youtube_read_scope")
@@ -138,7 +138,7 @@ def my_status(user: User = Depends(current_user), db: Session = Depends(get_db))
 
 @router.post("/deep-all")
 def deep_all(
-    user: User = Depends(require_human),
+    user: User = Depends(current_user),
     db: Session = Depends(get_db),
     on: bool = True,
 ) -> dict: