chore: fix eslint errors introduced in phase 4 backend

packages/backend/convex/maintenanceDedup.ts

@@ -16,6 +16,9 @@ export const maintenanceDedupKey = (input: {
 }): string | null => {
   const head = input.headSha?.trim();
   if (!head) return null;
+  // Intentional `||`: an empty/whitespace merge base must collapse to the
+  // 'nobase' sentinel, which `??` would not do (it keeps the empty string).
+  // eslint-disable-next-line @typescript-eslint/prefer-nullish-coalescing
   const base = input.mergeBaseSha?.trim() || 'nobase';
   return `${base}:${head}`;
 };