feat(db+auth): add pg pool, admin_users email/provider migration, refactor auth lookup

- Create src/lib/db.ts (shared pg.Pool, query/withTx helpers, server-only)
- Add @types/pg devDep
- Migration 204: add email, auth_provider, auth_subject columns to
  admin_users; backfill from auth.users; new upsert_admin_user accepts
  multi-provider args; new get_admin_user_for_session RPC resolves
  Auth.js session id (UUID or Google sub) to an admin row
- Refactor getAdminUser() to use pg + new RPC; auto-provisions on first
  Google sign-in using session.user.email
- Refactor updatePasswordAction to call update_user_password via pg
  (drops the Supabase REST hop)
- Delete orphaned src/actions/login.ts (replaced by auth-actions.ts)
- Drop remaining DEV_FORCE_UID references in users.ts; dev path now
  uses dev_session cookie (the only cookie the dev flow can set)
- Update AdminUser type: user_id is now string | null (Google users
  have no Supabase auth id); add email + auth_provider fields
- Fix downstream type errors: StopProductAssignment.callerUid accepts
  null; pickup.ts performedBy widens to string | null
- Bump vitest config, tests/, and other earlier cleanup changes

src/app/admin/settings/square-sync/page.tsx

@@ -4,6 +4,10 @@ import { getPaymentSettings } from "@/actions/payments";
 import { getSyncLog, type SyncLogEntry } from "@/actions/square-sync-ui";
 import SquareSyncSettingsClient from "./SquareSyncSettingsClient";
 
+// Uses cookies() via getAdminUser — must be dynamic to avoid the
+// "couldn't be rendered statically" build error.
+export const dynamic = "force-dynamic";
+
 export default async function SquareSyncSettingsPage() {
   const adminUser = await getAdminUser();
   if (!adminUser) redirect("/login");