feature: Phase 18.1 — derive Opus 320 + seek-index sidecar at ingest

Background-job transcode (ffmpeg/libopus) after source store; pure C# Ogg
walker builds the 0.5s-bucketed granule→byte seek index + captures the
OpusHead/OpusTags setup header into a per-track sidecar in a new track-opus
vault. Best-effort, additive, regenerated on replace-audio.

DeepDrftAPI/Startup.cs

@@ -4,6 +4,7 @@ using DeepDrftContent.Constants;
 using DeepDrftContent.FileDatabase.Models;
 using DeepDrftContent.FileDatabase.Services;
 using DeepDrftContent.Processors;
+using DeepDrftContent.Processors.Opus;
 using Microsoft.Extensions.Logging;
 using NetBlocks.Utilities.Environment;
 
@@ -44,6 +45,7 @@ namespace DeepDrftAPI
                 InitializeTrackVault(db).GetAwaiter().GetResult();
                 InitializeImageVault(db).GetAwaiter().GetResult();
                 InitializeTrackWaveformsVault(db).GetAwaiter().GetResult();
+                InitializeTrackOpusVault(db).GetAwaiter().GetResult();
                 return db;
             });
 
@@ -65,6 +67,16 @@ namespace DeepDrftAPI
             Environment.SetEnvironmentVariable("ASPNETCORE_TEMP", stagingPath);
             builder.Services.AddSingleton(new UploadStagingDirectory(stagingPath));
 
+            // Opus low-data transcode (Phase 18.1). The domain service lives in DeepDrftContent; the host
+            // owns only the engine config and the background worker. Bitrate/ffmpeg-path come from the
+            // OpusTranscode config section; StagingPath is forced to the same data-disk staging directory
+            // the upload path uses so large transcode temp files never land on the /tmp tmpfs.
+            builder.Services.Configure<OpusTranscodeOptions>(
+                builder.Configuration.GetSection(nameof(OpusTranscodeOptions)));
+            builder.Services.PostConfigure<OpusTranscodeOptions>(o => o.StagingPath = stagingPath);
+            builder.Services.AddSingleton<FfmpegOpusEncoder>();
+            builder.Services.AddSingleton<OpusTranscodeService>();
+
             return Task.CompletedTask;
         }
 
@@ -107,5 +119,15 @@ namespace DeepDrftAPI
                 await fileDatabase.CreateVaultAsync(VaultConstants.TrackWaveforms, MediaVaultType.Media);
             }
         }
+
+        // Ensure the track-opus vault exists (Phase 18.1). Holds the derived low-data Ogg Opus artifacts
+        // — the Opus audio bytes and the setup-header + seek-index sidecar — keyed by the track's EntryKey.
+        private static async Task InitializeTrackOpusVault(FileDatabase fileDatabase)
+        {
+            if (!fileDatabase.HasVault(VaultConstants.TrackOpus))
+            {
+                await fileDatabase.CreateVaultAsync(VaultConstants.TrackOpus, MediaVaultType.Audio);
+            }
+        }
     }
 }