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/config.py

@@ -146,8 +146,6 @@ class Settings(BaseSettings):
     autotag_interval_minutes: int = 30
     subscriptions_resync_minutes: int = 360
     playlist_sync_minutes: int = 360
-    # How often the shared demo account is auto-reset to a clean baseline (communal sandbox).
-    demo_reset_minutes: int = 720
     # --- Maintenance / validation job ---
     # Runs once a day by default (admin-tunable via the Scheduler dashboard). Grace periods
     # are in days because that's the granularity that matters; the rolling re-validation