feat(downloads): estimate a transcode's size so the progress bar moves

A transcode is produced as it is sent — chunked, with no Content-Length —
and the worker reported progress 0.0 for its whole duration: an empty bar
reading "0%" while the byte count climbed for an hour. That is the case
every film whose audio must be re-encoded lands in.

The backend already fetches the item to decide the audio policy, and that
item carries what a prediction needs: the source's size (an `original`
download copies the picture, so the output is the source give or take the
audio track) and its runtime (a preset re-encodes at fixed rates, so the
size is rate × runtime — from a preset table the URL builder now shares, so
the two cannot drift). The prediction is made where the URL is resolved and
persisted as the row's file_size. The worker uses it only when the response
has no length; the server's figure always wins; an estimated bar is capped
at 99% so a low prediction never shows a finished download still running;
and the Completed event now carries the bytes actually written so the
frontend stops persisting the row's file_size as the final size.

The row renders three honest states: exact "42%", estimated "~42%" with
"X / ~Y", or — with no total at all — an indeterminate band and the bytes
so far, never "0%". The single-video button joins the series/season buttons
on the enqueue path so all three resolve, and predict, in one place.

DR-290, UT-252, UT-253, UT-254.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/lib/stores/downloads.test.ts

@@ -568,6 +568,79 @@ describe("downloads store", () => {
       expect(state.downloads[123].status).toBe("completed");
     });
 
+    /// A transcode's total is a prediction. The bar must carry that so it can
+    /// say "~", and the row's persisted size on completion must be the bytes
+    /// the worker counted — never the prediction.
+    /// TRACES: UR-071 | DR-290 | UT-254
+    it("carries the estimate flag through progress and persists the real size on completion", async () => {
+      const { downloads, initDownloadEvents } = await import("./downloads");
+
+      mockInvoke.mockResolvedValueOnce({
+        downloads: [
+          {
+            id: 7,
+            itemId: "film",
+            userId: "user-1",
+            filePath: "videos/movies/film.mp4",
+            fileSize: 3_000_000_000, // predicted at resolve time
+            status: "downloading",
+            progress: 0,
+            bytesDownloaded: 0,
+            queuedAt: "2024-01-01T00:00:00Z",
+            retryCount: 0,
+            priority: 0,
+            mediaType: "video",
+            downloadSource: "user",
+          },
+        ],
+        stats: {
+          total: 1,
+          activeCount: 1,
+          queuedCount: 0,
+          completedCount: 0,
+          failedCount: 0,
+          pausedCount: 0,
+        },
+      });
+      await downloads.refresh("user-1");
+      await initDownloadEvents();
+
+      eventHandler!({
+        payload: {
+          type: "progress",
+          downloadId: 7,
+          itemId: "film",
+          bytesDownloaded: 1_500_000_000,
+          totalBytes: 3_000_000_000,
+          progress: 0.5,
+          estimated: true,
+        },
+      });
+      let row = get(downloads).downloads[7];
+      expect(row.fileSizeEstimated).toBe(true);
+      expect(row.progress).toBe(0.5);
+
+      mockInvoke.mockClear();
+      mockInvoke.mockResolvedValueOnce(undefined);
+      eventHandler!({
+        payload: {
+          type: "completed",
+          downloadId: 7,
+          itemId: "film",
+          filePath: "videos/movies/film.mp4",
+          bytesDownloaded: 3_120_000_000,
+        },
+      });
+      await new Promise((resolve) => setTimeout(resolve, 50));
+
+      row = get(downloads).downloads[7];
+      expect(row.status).toBe("completed");
+      expect(row.fileSizeEstimated).toBe(false);
+      expect(row.fileSize).toBe(3_120_000_000);
+      const persisted = mockInvoke.mock.calls.find((c) => c[0] === "mark_download_completed");
+      expect(persisted?.[1]).toMatchObject({ bytesDownloaded: 3_120_000_000 });
+    });
+
     it("should handle failed event and refresh", async () => {
       const { downloads, initDownloadEvents } = await import("./downloads");
 

src/lib/stores/downloads.ts

@@ -17,6 +17,8 @@ export interface DownloadInfo {
   userId: string;
   filePath: string;
   fileSize?: number;
+  /** `fileSize` is the backend's prediction (a transcode states no length), not the server's word. */
+  fileSizeEstimated?: boolean;
   mimeType?: string;
   status: "pending" | "downloading" | "completed" | "failed" | "paused";
   progress: number;
@@ -58,6 +60,8 @@ export interface DownloadEvent {
   bytesDownloaded?: number;
   totalBytes?: number;
   progress?: number;
+  /** On 'progress': `totalBytes` is an estimate, so `progress` stays below 1 until 'completed'. */
+  estimated?: boolean;
   filePath?: string;
   error?: string;
 }
@@ -598,17 +602,24 @@ function handleDownloadEvent(payload: DownloadEvent): void {
           progress: payload.progress,
           bytesDownloaded: payload.bytesDownloaded || download.bytesDownloaded,
           fileSize: payload.totalBytes || download.fileSize,
+          fileSizeEstimated: payload.estimated ?? download.fileSizeEstimated ?? false,
         });
       }
       break;
 
     case "completed":
       if (download) {
+        // The bytes actually written, as the worker counted them. A predicted
+        // fileSize must never be persisted as the real one (DR-290).
+        const finalBytes =
+          payload.bytesDownloaded ||
+          (download.fileSizeEstimated ? 0 : download.fileSize) ||
+          download.bytesDownloaded;
         // Persist to database
         commands
           .markDownloadCompleted(
             payload.downloadId,
-            payload.totalBytes || download.fileSize || download.bytesDownloaded,
+            finalBytes,
             payload.filePath || download.filePath,
           )
           .catch((err) => log.error("Failed to persist download completion:", err));
@@ -616,6 +627,9 @@ function handleDownloadEvent(payload: DownloadEvent): void {
         updateDownloadInStore(payload.downloadId, {
           status: "completed",
           progress: 1.0,
+          bytesDownloaded: finalBytes,
+          fileSize: finalBytes,
+          fileSizeEstimated: false,
           completedAt: new Date().toISOString(),
           filePath: payload.filePath || download.filePath,
         });