feat(demo): demo-account schema + reusable rate limiter

Add the shared demo-account plumbing: users.is_demo marks the single
shared demo user, demo_whitelist holds the admin-curated emails that may
enter it without Google sign-in, and a small in-process RateLimiter
(generic groundwork) for throttling the demo-login endpoint per IP.

backend/alembic/versions/0014_demo_account.py

@@ -0,0 +1,59 @@
+"""demo account: users.is_demo + demo_whitelist
+
+Revision ID: 0014_demo_account
+Revises: 0013_playlist_fingerprint
+Create Date: 2026-06-16
+
+Adds the shared demo-account plumbing:
+  - users.is_demo — marks the single shared demo user (no OAuth token / YouTube scope).
+  - demo_whitelist — admin-curated emails that may enter the demo account from the login
+    page without Google sign-in. Multiple emails are just keys to the same shared door.
+The demo user row itself is created lazily on first demo login (works across all three DBs
+without a data migration), so this only adds the schema.
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0014_demo_account"
+down_revision: Union[str, None] = "0013_playlist_fingerprint"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> 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
+    )
+
+
+def downgrade() -> None:
+    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")