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/web/api/audit.test.ts

@@ -20,7 +20,7 @@ describe("audit router", () => {
     const users = createUserStore(botDb.db);
     const sessions = createSessionStore(botDb.db);
     const audit = createAuditStore(botDb.db);
-    const alice = await users.createUser("alice", "pw-alice");
+    const alice = await users.createUser("alice", "pw-alice", "admin");
     cookie = `${SESSION_COOKIE_NAME}=${sessions.createSession(alice.id).token}`;
     for (let i = 0; i < 3; i++) {
       audit.record({