fix(backfill): type spec queue mock so production nest build compiles it

The prod docker build runs `nest build`, which compiles spec files too —
the untyped `vi.fn(async () => undefined)` mock made `queue.add.mock.calls[i]`
a zero-length tuple, so the destructures tripped TS2493/TS2352 and failed
`pnpm build` (tsc --noEmit had skipped the spec). Type the mock args as
unknown[]. No behaviour change; tests still 7/7, `pnpm build` green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

apps/api/src/jobs/prefetch-worker.service.spec.ts

@@ -18,7 +18,9 @@ function makeDb(limitResults: unknown[][]) {
 
 function makeDeps(opts: { waiting: number; limitResults: unknown[][] }) {
   const queue = {
-    add: vi.fn(async () => undefined),
+    // typed args so `.mock.calls[i]` is `unknown[]` (not a 0-length tuple) — the
+    // production `nest build` compiles spec files and rejects tuple-index access.
+    add: vi.fn((..._args: unknown[]) => Promise.resolve(undefined)),
     getJobCounts: vi.fn(async () => ({ waiting: opts.waiting, delayed: 0, active: 0 })),
   };
   const redis = {