fix: Wave 18.1 review — pre-skip subtraction, t=0 anchor, PreSkip in sidecar, stderr on cancel

DeepDrftContent/Processors/Opus/OpusTranscodeService.cs

@@ -132,6 +132,9 @@ public sealed class OpusTranscodeService
 
     private async Task EnsureVaultAsync()
     {
+        // The TrackOpus vault is created at host startup (Startup.cs), so this guard is normally a
+        // no-op for the upload path. It is retained for the backfill path, which may run via a
+        // standalone CLI or a host that skips vault pre-creation, where the vault might not exist.
         if (!_fileDatabase.HasVault(VaultConstants.TrackOpus))
             await _fileDatabase.CreateVaultAsync(VaultConstants.TrackOpus, MediaVaultType.Audio);
     }