feat: replace ?offset= seek with HTTP Range streaming across API, proxy, and client

- API: enableRangeProcessing true on no-offset FileStream path
- Proxy: transparent Range relay, forwards 206/416/Content-Range verbatim
- TrackMediaClient: Range: bytes=X- replaces ?offset=X; response disposed via TrackMediaResponse
- StreamDecoder: reinitializeForRangeContinuation retains wavHeader, counts raw PCM against 206 Content-Length
- AudioPlayer: seekBeyondBuffer adds headerSize for file-absolute offset; duration guard prevents continuation overwriting full-track duration
- StreamingAudioPlayerService: seek guard corrected to >= 0 (file-absolute offset contract)

DeepDrftPublic.Client/Clients/TrackMediaClient.cs

@@ -1,4 +1,5 @@
 using System.Net;
+using System.Net.Http.Headers;
 using System.Net.Http.Json;
 using DeepDrftModels.DTOs;
 using Microsoft.Extensions.DependencyInjection;
@@ -10,16 +11,19 @@ public class TrackMediaResponse : IDisposable
 {
     public Stream Stream { get; }
     public long ContentLength { get; }
+    private readonly HttpResponseMessage _response;
 
-    public TrackMediaResponse(Stream stream, long contentLength)
+    public TrackMediaResponse(Stream stream, long contentLength, HttpResponseMessage response)
     {
         Stream = stream;
         ContentLength = contentLength;
+        _response = response;
     }
 
     public void Dispose()
     {
         Stream?.Dispose();
+        _response?.Dispose();
     }
 }
 
@@ -33,10 +37,12 @@ public class TrackMediaClient
     }
 
     /// <summary>
-    /// Fetches the WAV stream for a track, optionally starting from a byte offset.
-    /// The cancellation token is forwarded to <see cref="HttpClient.GetAsync"/> so a
-    /// navigation or seek-replacement aborts the in-flight server connection rather
-    /// than leaving the server draining bytes into a dead socket.
+    /// Fetches the WAV stream for a track via an HTTP Range request starting at a
+    /// file-absolute byte offset. <paramref name="byteOffset"/> is the position from
+    /// the start of the file on disk (including the WAV header) — callers seeking into
+    /// audio data must add the header size themselves. The cancellation token aborts
+    /// the in-flight server connection rather than leaving the server draining bytes
+    /// into a dead socket.
     /// </summary>
     public async Task<ApiResult<TrackMediaResponse>> GetTrackMedia(
         string trackId,
@@ -45,19 +51,21 @@ public class TrackMediaClient
     {
         try
         {
-            // Build URL with optional offset parameter
-            var url = byteOffset > 0
-                ? $"api/track/{trackId}?offset={byteOffset}"
-                : $"api/track/{trackId}";
+            // Same URL for every seek — only the Range header differs. byteOffset 0 is
+            // not special-cased: "bytes=0-" requests the whole file from the start.
+            using var request = new HttpRequestMessage(HttpMethod.Get, $"api/track/{trackId}");
+            request.Headers.Range = new RangeHeaderValue(byteOffset, null);
 
             // Use HttpCompletionOption.ResponseHeadersRead to get stream immediately
-            var response = await _http.GetAsync(url, HttpCompletionOption.ResponseHeadersRead, cancellationToken);
+            var response = await _http.SendAsync(request, HttpCompletionOption.ResponseHeadersRead, cancellationToken);
             response.EnsureSuccessStatusCode();
 
             var contentLength = response.Content.Headers.ContentLength ?? 0;
             var stream = await response.Content.ReadAsStreamAsync(cancellationToken);
 
-            return ApiResult<TrackMediaResponse>.CreatePassResult(new TrackMediaResponse(stream, contentLength));
+            // TrackMediaResponse takes ownership of both stream and response;
+            // do NOT dispose response here — the caller disposes via TrackMediaResponse.Dispose().
+            return ApiResult<TrackMediaResponse>.CreatePassResult(new TrackMediaResponse(stream, contentLength, response));
         }
         catch (Exception e)
         {