feat(auth): two-role permission system (admin/member)

Adds an admin/member role to WebUI auth. /api/users and /api/audit
are now gated by a requireAdmin middleware; all other authenticated
endpoints accept both roles. Schema migration defaults all existing
users to admin to preserve access. POST /api/users defaults new users
to member; first-run setup always creates an admin. Adds PATCH
/api/users/:id/role with last-admin demotion and deletion guards.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/data/audit.ts

@@ -5,7 +5,8 @@ export type AuditAction =
   | "user.created"
   | "user.deleted"
   | "user.password_reset"
-  | "user.password_changed";
+  | "user.password_changed"
+  | "user.role_changed";
 
 export interface AuditEntry {
   id: number;