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.

scripts/e2e_seed.py

@@ -4,7 +4,7 @@ Run INSIDE the api container — it needs the app's models + the argon2 hasher:
 
     docker exec -i siftlode-api-1 python - < scripts/e2e_seed.py
 
-Resets a dedicated, NON-demo test account to a known baseline every run, so the Playwright
+Resets a dedicated test account to a known baseline every run, so the Playwright
 golden-master net is deterministic and never touches a real account. Self-healing: a crashed
 spec that left the account dirty is cleaned on the next run. Prints one JSON line describing
 what it seeded (the wrapper / global-setup reads it).
@@ -68,7 +68,7 @@ def main() -> None:
         db.commit()
 
         # 1. Upsert the account and always reset it to a clean, loginable baseline
-        #    (email_verified + is_active + not suspended + not demo + password set).
+        #    (email_verified + is_active + not suspended + password set).
         user = db.scalar(select(User).where(User.email == EMAIL))
         if user is None:
             user = User(email=EMAIL)
@@ -77,7 +77,6 @@ def main() -> None:
         user.email_verified = True
         user.is_active = True
         user.is_suspended = False
-        user.is_demo = False
         user.google_sub = None
         # Admin so the E2E can reach the admin-only pages (scheduler / config / users / audit) and
         # thus the render ternary's admin branches — the primary real user (npeter83) is admin too.