feat(admin): user management — roles, suspend, delete + lifecycle emails

- New Users page tabs (Users & roles / Access requests / Demo) and a tab-ified Configuration
  page, both via a reusable Tabs component (persisted active tab).
- Admin can suspend/unsuspend (migration 0023 adds users.is_suspended) and delete accounts
  from the Users & roles tab, with guards (demo / self / last admin).
- Email notifications to the affected user: suspended (reactive, on a blocked sign-in),
  reinstated, role changed, and account deleted; the approval email now carries a clickable
  app link. The scheduled/manual demo reset also seeds sample channel subscriptions.
- Hide the 'unverified email' chip for Google accounts (Google attests the email).

backend/alembic/versions/0023_user_suspension.py

@@ -0,0 +1,30 @@
+"""admin account suspension
+
+Revision ID: 0023_user_suspension
+Revises: 0022_password_auth
+Create Date: 2026-06-19
+
+Adds users.is_suspended — an admin-controlled access block, distinct from is_active (the
+approval lifecycle). A suspended account can't sign in (password or Google) and any live
+session is rejected; the user is told why and given the operator's contact.
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0023_user_suspension"
+down_revision: Union[str, None] = "0022_password_auth"
+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_suspended", sa.Boolean(), nullable=False, server_default="false"),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("users", "is_suspended")