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/models.py

@@ -106,7 +106,7 @@ class User(Base, TimestampMixin, CanonicalEmailMixin):
     # half of the truth lives, for the UI to show when the two differ. NULL until the next Google
     # sign-in for accounts linked before this column existed.
     google_email: Mapped[str | None] = mapped_column(String(320))
-    # argon2id hash for email+password login; NULL for Google-only / demo accounts.
+    # argon2id hash for email+password login; NULL for Google-only accounts.
     password_hash: Mapped[str | None] = mapped_column(String(255))
     # When SEC-10 dropped a preset password on Google adoption (see the single caller of
     # auth.revoke_preset_password_if_unverified; admin approval no longer revokes — E9.5 S1). Without
@@ -123,7 +123,7 @@ class User(Base, TimestampMixin, CanonicalEmailMixin):
     is_active: Mapped[bool] = mapped_column(Boolean, default=True, server_default="true")
     # Admin-imposed access block, distinct from is_active (approval lifecycle). A suspended
     # account can't sign in by any method and its live sessions are rejected; the user is told
-    # why and pointed at the operator's contact. The demo account is never suspendable.
+    # why and pointed at the operator's contact.
     is_suspended: Mapped[bool] = mapped_column(Boolean, default=False, server_default="false")
     # Server-side session revocation (SA4): a monotonic counter bumped on password reset, password
     # change, or "log out everywhere". A signed session cookie records the epoch it was minted under;
@@ -132,7 +132,7 @@ class User(Base, TimestampMixin, CanonicalEmailMixin):
     session_epoch: Mapped[int] = mapped_column(Integer, default=0, server_default="0")
     # When this account last signed in (Google or password), stamped by the one sign-in path,
     # routes/auth._establish_session. NULL = not since the column existed (0085; no backfill is
-    # possible). The shared demo account is never stamped — its "last login" would mean nobody.
+    # possible).
     last_login_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
     # The diagnostics level this user consented to (E22 S5) and when it lapses back to the base
     # level. Both NULL = base. Written only by `PUT /api/me/diagnostics`, which computes the expiry
@@ -146,12 +146,6 @@ class User(Base, TimestampMixin, CanonicalEmailMixin):
     # The user's own YouTube channel id, resolved lazily (channels.list?mine=true) and
     # cached. Used to exclude their own channel from playlist discovery. NULL = unresolved.
     yt_channel_id: Mapped[str | None] = mapped_column(String(32))
-    # The single shared "demo" account: signed into without Google OAuth (via a whitelisted
-    # email on the login page), has no OAuth token / YouTube scope, and its per-user state is
-    # shared by everyone who enters this way. Exactly one such row is expected.
-    is_demo: Mapped[bool] = mapped_column(
-        Boolean, default=False, server_default="false", index=True
-    )
     # Free-form UI preferences (theme, color scheme, font scale, default filters…).
     preferences: Mapped[dict | None] = mapped_column(JSON)
 
@@ -224,19 +218,6 @@ class Invite(Base, CanonicalEmailMixin):
     lang: Mapped[str | None] = mapped_column(String(8))
 
 
-class DemoWhitelist(Base, TimestampMixin):
-    """Emails that may enter the shared demo account from the login page (no Google OAuth).
-    A hidden, admin-curated list of "keys to the same door" — every entry logs into the one
-    shared demo user. Distinct from Invite (which gates real Google sign-in)."""
-
-    __tablename__ = "demo_whitelist"
-
-    id: Mapped[int] = mapped_column(primary_key=True)
-    email: Mapped[str] = mapped_column(String(320), unique=True, index=True)
-    note: Mapped[str | None] = mapped_column(Text)
-    added_by: Mapped[str | None] = mapped_column(String(320))  # admin email
-
-
 class Channel(Base, TimestampMixin):
     """A YouTube channel. Shared across all users (one channel's videos are the same
     for everyone), so its expensive metadata is fetched and stored only once."""