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/userscope.py

@@ -1,23 +1,22 @@
-"""Shared "who is a real, active, non-suspended human" predicate.
+"""Shared "who is an active, non-suspended account" predicate.
 
-Used wherever a user must be filtered out of a directory / recipient list or rejected as a target:
-the messaging directory AND the download share-by-email / recipient-picker endpoints. Kept in one
-place so the two forms (SQL clauses for a query, Python check for a loaded row) can't drift apart,
-and so a route module doesn't have to import another route module just to reuse the rule.
+Used wherever a user must be filtered out of a recipient list or rejected as a target: the download
+share-by-email and recipient-picker endpoints. Kept in one place so the two forms (SQL clauses for a
+query, Python check for a loaded row) can't drift apart, and so a route module doesn't have to import
+another route module just to reuse the rule.
 """
 
 from app.models import User
 
 
 def messageable_clauses() -> list:
-    """SQL clauses for "a real, active, non-suspended human" — splat into a directory/recipient query."""
+    """SQL clauses for "an active, non-suspended account" — splat into a recipient query."""
     return [
-        User.is_demo.is_(False),
         User.is_active.is_(True),
         User.is_suspended.is_(False),
     ]
 
 
 def is_messageable_user(u: User | None) -> bool:
-    """Python mirror of `messageable_clauses()` for an already-loaded row (WS auth, send recipient)."""
-    return bool(u and not u.is_demo and u.is_active and not u.is_suspended)
+    """Python mirror of `messageable_clauses()` for an already-loaded row (a share recipient)."""
+    return bool(u and u.is_active and not u.is_suspended)