Wire Opus end-to-end playback + Backfill-Opus action (Phase 18.5)

Player picks Opus when the browser can decode it and a sidecar exists (else lossless), injecting the sidecar before stream init; seek reuses the same format. Adds the Backfill-Opus bulk API endpoint + CMS action.

DeepDrftManager/Services/CmsTrackService.cs

@@ -765,6 +765,45 @@ public class CmsTrackService : ICmsTrackService
         }
     }
 
+    public async Task<ResultContainer<OpusBackfillResult>> BackfillOpusAsync(CancellationToken ct = default)
+    {
+        var client = _httpClientFactory.CreateClient(ContentCmsClientName);
+
+        HttpResponseMessage response;
+        try
+        {
+            response = await client.PostAsync("api/track/opus/backfill", null, ct);
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(ex, "Content API call failed for Opus backfill");
+            return ResultContainer<OpusBackfillResult>.CreateFailResult("Content API is unreachable.");
+        }
+
+        using (response)
+        {
+            if (!response.IsSuccessStatusCode)
+            {
+                var body = await response.Content.ReadAsStringAsync(ct);
+                _logger.LogError("Content API Opus backfill failed: {Status} {Body}", (int)response.StatusCode, body);
+                return ResultContainer<OpusBackfillResult>.CreateFailResult("Failed to start the Opus backfill.");
+            }
+
+            OpusBackfillResult payload;
+            try
+            {
+                payload = await response.Content.ReadFromJsonAsync<OpusBackfillResult>(ct);
+            }
+            catch (Exception ex)
+            {
+                _logger.LogError(ex, "Failed to deserialize Opus backfill response from Content API");
+                return ResultContainer<OpusBackfillResult>.CreateFailResult("Content API returned an unexpected response.");
+            }
+
+            return ResultContainer<OpusBackfillResult>.CreatePassResult(payload);
+        }
+    }
+
     public async Task<ResultContainer<List<ReleaseDto>>> GetReleasesAsync(CancellationToken ct = default)
     {
         var client = _httpClientFactory.CreateClient(ContentCmsClientName);

DeepDrftManager/Services/ICmsTrackService.cs

@@ -152,6 +152,15 @@ public interface ICmsTrackService
     /// </summary>
     Task<Result> GenerateHighResWaveformAsync(string entryKey, CancellationToken ct = default);
 
+    /// <summary>
+    /// Trigger the catalogue-wide Backfill-Opus pass via <c>POST api/track/opus/backfill</c> (Phase 18.5).
+    /// The API enqueues a background Opus derive for every track lacking a complete Opus artifact and returns
+    /// the (enqueued, skipped) counts. Enqueue-only — the transcodes run server-side on a serial background
+    /// worker, so this call returns as soon as the work is scheduled, not when transcoding finishes. The
+    /// <c>Enqueued</c> count is how many derives were scheduled; <c>Skipped</c> is how many already had Opus.
+    /// </summary>
+    Task<ResultContainer<OpusBackfillResult>> BackfillOpusAsync(CancellationToken ct = default);
+
     /// <summary>Returns all releases with track counts from GET api/track/albums.</summary>
     Task<ResultContainer<List<ReleaseDto>>> GetReleasesAsync(CancellationToken ct = default);
 
@@ -160,3 +169,11 @@ public interface ICmsTrackService
     /// </summary>
     Task<ResultContainer<int>> GetTrackCountAsync(CancellationToken ct = default);
 }
+
+/// <summary>
+/// Outcome of a Backfill-Opus pass (Phase 18.5): how many tracks had a background derive scheduled
+/// (<paramref name="Enqueued"/>) and how many were skipped because they already carry a complete Opus
+/// artifact (<paramref name="Skipped"/>). Both are counts of tracks, not finished transcodes — the work
+/// runs asynchronously on the API's background worker after this returns.
+/// </summary>
+public readonly record struct OpusBackfillResult(int Enqueued, int Skipped);