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/alembic/versions/0098_drop_demo_account.py

@@ -0,0 +1,55 @@
+"""demo: drop the shared demo account (the owner's decision, 2026-10-04)
+
+The demo account — one shared user that whitelisted emails entered from the login page, reset on a
+schedule — goes with its code. Prod held the demo user and 10 whitelist rows.
+
+The upgrade deletes the demo user first (its playlists, states and subscriptions cascade; audit and
+quota rows keep their history with the actor nulled, as for any deleted account), then the traces
+that only the demo code could render: its audit rows (whitelist edits, manual resets, the scheduled
+reset's job runs), the reset job's interval override and its completion notices. Nothing is
+migrated anywhere; the nightly backup holds the rows.
+
+The downgrade restores the schema as it stood (0014), empty: the demo user is created lazily on its
+first sign-in, so an empty `demo_whitelist` is the whole of its state.
+"""
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0098_drop_demo_account"
+down_revision: str | None = "0097_drop_messages"
+branch_labels: str | None = None
+depends_on: str | None = None
+
+
+def upgrade() -> None:
+    op.execute("DELETE FROM users WHERE is_demo")
+    op.execute(
+        "DELETE FROM audit_log WHERE action IN ('demo_add', 'demo_remove', 'demo_reset')"
+        " OR (action = 'job_run' AND target_id = 'demo_reset')"
+    )
+    op.execute("DELETE FROM scheduler_settings WHERE job_id = 'demo_reset'")
+    op.execute("DELETE FROM notifications WHERE data->>'job_id' = 'demo_reset'")
+    op.drop_index("ix_demo_whitelist_email", table_name="demo_whitelist")
+    op.drop_table("demo_whitelist")
+    op.drop_index("ix_users_is_demo", table_name="users")
+    op.drop_column("users", "is_demo")
+
+
+def downgrade() -> None:
+    op.add_column(
+        "users",
+        sa.Column("is_demo", sa.Boolean(), nullable=False, server_default=sa.false()),
+    )
+    op.create_index("ix_users_is_demo", "users", ["is_demo"])
+    op.create_table(
+        "demo_whitelist",
+        sa.Column("id", sa.Integer(), primary_key=True),
+        sa.Column("email", sa.String(length=320), nullable=False),
+        sa.Column("note", sa.Text(), nullable=True),
+        sa.Column("added_by", sa.String(length=320), nullable=True),
+        sa.Column(
+            "created_at", sa.DateTime(timezone=True), server_default=sa.func.now(), nullable=False
+        ),
+    )
+    op.create_index("ix_demo_whitelist_email", "demo_whitelist", ["email"], unique=True)