feat: track download progress on Job (migration + worker poller + API)

Add Job.downloadProgress (0.0-1.0), derived from a worker background
poller that counts audio files landing in staging / expected track
count, since streamrip only reports 0/1. No UI yet (slice B).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/app/api/requests/route.test.ts

@@ -62,7 +62,15 @@ describe("requests API", () => {
       data: {
         artist: "Chosen Artist",
         album: "Chosen Album",
-        job: { create: { state: "downloading", currentStage: "download", attempts: 2, error: "boom" } },
+        job: {
+          create: {
+            state: "downloading",
+            currentStage: "download",
+            attempts: 2,
+            error: "boom",
+            downloadProgress: 0.42,
+          },
+        },
       },
       include: { job: true },
     });
@@ -99,6 +107,7 @@ describe("requests API", () => {
     expect(found.job.chosen).toEqual({ source: "qobuz", format: "flac", trackCount: 12 });
     expect(found.job.error).toBe("boom");
     expect(found.job.attempts).toBe(2);
+    expect(found.job.downloadProgress).toBe(0.42);
   });
 
   it("reports no chosen candidate and zero count when a job has none", async () => {
@@ -109,5 +118,6 @@ describe("requests API", () => {
     const found = body.requests.find((r: { id: string }) => r.id === created.id);
     expect(found.job.candidateCount).toBe(0);
     expect(found.job.chosen).toBeNull();
+    expect(found.job.downloadProgress).toBe(0);
   });
 });

web/src/app/api/requests/route.ts

@@ -63,6 +63,7 @@ export async function GET() {
               error: r.job.error,
               claimedAt: r.job.claimedAt,
               updatedAt: r.job.updatedAt,
+              downloadProgress: r.job.downloadProgress,
               candidateCount: cands.length,
               chosen: chosen ? { source: chosen.source, format: chosen.format, trackCount: chosen.trackCount } : null,
             };