fix(api): stream audio store path to eliminate whole-file buffering (OOM)

Processors now emit a ProcessedAudio plan with a streamed writer instead of a whole-file AudioBinary; vault writes stream via RegisterResourceStreamingAsync. Header parsing is bounded. Wave 2 (waveform/Opus) still re-reads the full file by design.

DeepDrftContent/Processors/ProcessedAudio.cs

@@ -0,0 +1,68 @@
+namespace DeepDrftContent.Processors;
+
+/// <summary>
+/// The product of processing an uploaded audio file on the store path: the metadata SQL and the
+/// vault index need, plus a streamed writer that emits the canonical vault bytes to a destination
+/// stream without ever materializing the whole file in a managed <c>byte[]</c>.
+///
+/// This replaces the former whole-file <c>AudioBinary</c> as the processor output for upload /
+/// replace-audio (Wave 1 OOM fix): passthrough formats (standard-PCM WAV, MP3, FLAC) stream the
+/// source file straight to the destination, and EXTENSIBLE WAVs stream their normalization to
+/// standard PCM. The vault <em>load</em> path still uses <c>AudioBinary</c> (a full buffer) — that
+/// is the Wave 2 read path and is out of scope here.
+///
+/// <see cref="WriteToAsync"/> is invoked exactly once by the streaming vault register, against the
+/// freshly opened backing <see cref="System.IO.FileStream"/>. The writer re-opens the source file
+/// itself, so the source (a staging file) must still exist when the register runs — it does, because
+/// processing and registration are sequential within the store call, before the staging-file
+/// <c>finally</c> cleanup.
+/// </summary>
+public sealed class ProcessedAudio
+{
+    /// <summary>The stored file extension (e.g. <c>.wav</c>, <c>.mp3</c>, <c>.flac</c>).</summary>
+    public string Extension { get; }
+
+    /// <summary>Audio duration in seconds, extracted from the header.</summary>
+    public double Duration { get; }
+
+    /// <summary>Audio bitrate in kbps, extracted from (or estimated for) the header.</summary>
+    public int Bitrate { get; }
+
+    /// <summary>
+    /// The canonical stored byte count — computed from the header and file length, never by
+    /// buffering the body. Used only for diagnostics (confirming the streamed path was taken).
+    /// </summary>
+    public long Size { get; }
+
+    private readonly Func<Stream, CancellationToken, Task> _writeTo;
+
+    public ProcessedAudio(
+        string extension,
+        double duration,
+        int bitrate,
+        long size,
+        Func<Stream, CancellationToken, Task> writeTo)
+    {
+        Extension = extension;
+        Duration = duration;
+        Bitrate = bitrate;
+        Size = size;
+        _writeTo = writeTo;
+    }
+
+    /// <summary>
+    /// Streams the canonical vault bytes to <paramref name="destination"/>. Bounded-buffer — peak
+    /// managed memory is O(buffer), not O(filesize).
+    /// </summary>
+    public Task WriteToAsync(Stream destination, CancellationToken cancellationToken = default)
+        => _writeTo(destination, cancellationToken);
+
+    /// <summary>
+    /// Builds a passthrough plan: the stored bytes are byte-identical to the source file (standard
+    /// PCM WAV, MP3, FLAC — no transcoding). The writer is a bounded disk-to-disk copy.
+    /// </summary>
+    public static ProcessedAudio Passthrough(
+        string sourcePath, string extension, double duration, int bitrate, long sourceLength)
+        => new(extension, duration, bitrate, sourceLength,
+            (destination, ct) => AudioStoreStream.CopyFileAsync(sourcePath, destination, ct));
+}