fix(api): import isNull from @groombook/db instead of drizzle-orm directly

drizzle-orm is not a direct dependency of @groombook/api, causing
TS2307 at typecheck time. Re-export isNull from @groombook/db and
update the import in rbac.ts.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/api/src/middleware/rbac.ts

@@ -1,6 +1,5 @@
 import type { MiddlewareHandler } from "hono";
-import { isNull } from "drizzle-orm";
-import { and, eq, getDb, staff } from "@groombook/db";
+import { and, eq, getDb, isNull, staff } from "@groombook/db";
 
 export type StaffRole = "groomer" | "receptionist" | "manager";
 export type StaffRow = typeof staff.$inferSelect;