feat(cms): replace track audio in edit form, gate last-track delete

Swap a track's audio by EntryKey (metadata/release/position preserved, waveform regenerated); hide per-track remove on a release's sole persisted track so it can only be replaced or release-deleted.

DeepDrftAPI/Controllers/TrackController.cs

@@ -479,6 +479,86 @@ public class TrackController : ControllerBase
         return StatusCode(500, error);
     }
 
+    // POST api/track/{id}/replace-audio ([ApiKeyAuthorize])
+    // Swap an existing track's audio bytes from a raw upload, preserving the track's id, EntryKey,
+    // release membership, position, and metadata. UnifiedTrackService.ReplaceAudioAsync owns the
+    // vault swap + waveform regen; nothing in SQL is written. Mirrors the upload endpoint's temp-file
+    // streaming and 1 GB ceiling (a WAV replace is a large-body upload like the original). The
+    // literal "{id:long}/replace-audio" segment is declared in the literal-route block so it never
+    // resolves to the parameterized "{trackId}" GET.
+    [ApiKeyAuthorize]
+    [HttpPost("{id:long}/replace-audio")]
+    [RequestSizeLimit(1_073_741_824)]
+    [RequestFormLimits(MultipartBodyLengthLimit = 1_073_741_824)]
+    public async Task<ActionResult> ReplaceAudio(
+        long id,
+        [FromForm] IFormFile? audioFile,
+        CancellationToken cancellationToken)
+    {
+        _logger.LogInformation("ReplaceAudio called: id={Id}, size={Size}", id, audioFile?.Length);
+
+        if (audioFile is null || audioFile.Length == 0)
+        {
+            return BadRequest("Audio file is required");
+        }
+
+        var uploadExtension = Path.GetExtension(audioFile.FileName).ToLowerInvariant();
+        if (uploadExtension is not (".wav" or ".mp3" or ".flac"))
+        {
+            return BadRequest("Uploaded file must have a .wav, .mp3, or .flac extension");
+        }
+
+        // The processor router selects by extension and reads from disk, so the temp file must carry
+        // the upload's real extension. Mirrors UploadTrack — Path.GetTempFileName() yields .tmp.
+        var tempPath = Path.Combine(Path.GetTempPath(), Guid.NewGuid().ToString("N") + uploadExtension);
+
+        try
+        {
+            await using (var tempStream = new FileStream(
+                tempPath, FileMode.CreateNew, FileAccess.Write, FileShare.None,
+                bufferSize: 81920, useAsync: true))
+            await using (var uploadStream = audioFile.OpenReadStream())
+            {
+                await uploadStream.CopyToAsync(tempStream, cancellationToken);
+            }
+
+            var result = await _unifiedService.ReplaceAudioAsync(id, tempPath, cancellationToken);
+            if (result.Success)
+            {
+                _logger.LogInformation("ReplaceAudio succeeded: id={Id}", id);
+                return Ok();
+            }
+
+            var error = result.Messages.FirstOrDefault()?.Message ?? "Failed to replace audio";
+            if (string.Equals(error, UnifiedTrackService.TrackNotFoundMessage, StringComparison.Ordinal))
+            {
+                return NotFound();
+            }
+
+            _logger.LogError("ReplaceAudio failed for id {Id}: {Error}", id, error);
+            return StatusCode(500, error);
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(ex, "ReplaceAudio failed for id {Id}", id);
+            return StatusCode(500, "Internal server error");
+        }
+        finally
+        {
+            try
+            {
+                if (System.IO.File.Exists(tempPath))
+                {
+                    System.IO.File.Delete(tempPath);
+                }
+            }
+            catch (Exception ex)
+            {
+                _logger.LogWarning(ex, "ReplaceAudio: failed to delete temp file {TempPath}", tempPath);
+            }
+        }
+    }
+
     // DELETE api/track/release/{id} ([ApiKeyAuthorize])
     // Soft-delete a release row directly. Used by the albums browser to remove an orphaned release
     // (one with no live tracks). "release" is a literal segment, declared here in the literal-route