Add server-side waveform loudness profiling on track upload

ILoudnessAlgorithm strategy (RmsLoudnessAlgorithm first impl), WaveformProfileService
stores quantized byte[] sidecar in new MediaFileVault (profiles vault), wired into
UnifiedTrackService.UploadAsync; failure is logged and swallowed. WaveformProfileDto
and WaveformProfileOptions in shared projects.

DeepDrftContent/Processors/ILoudnessAlgorithm.cs

@@ -0,0 +1,23 @@
+namespace DeepDrftContent.Processors;
+
+/// <summary>
+/// Strategy for reducing a stream of PCM samples to a fixed-length, peak-normalized loudness
+/// envelope. Swappable so the loudness measure (RMS today, LUFS later) can change without
+/// touching <c>WaveformProfileService</c>, the stored wire format, or the frontend renderer.
+/// </summary>
+public interface ILoudnessAlgorithm
+{
+    /// <summary>
+    /// Computes a peak-normalized loudness profile from raw interleaved PCM.
+    /// </summary>
+    /// <param name="pcmData">Interleaved, little-endian PCM sample bytes (the WAV data chunk).</param>
+    /// <param name="channels">Number of interleaved channels; averaged to mono per sample.</param>
+    /// <param name="sampleRate">Samples per second (unused by RMS but part of the contract for measures that need it).</param>
+    /// <param name="bitsPerSample">Bit depth (8 unsigned, 16/24/32 signed) used to decode samples.</param>
+    /// <param name="bucketCount">Number of equal time slices to reduce the signal to.</param>
+    /// <returns>
+    /// A <c>double[bucketCount]</c>, each value in [0, 1], peak-normalized so the loudest bucket
+    /// is 1. All zeros when the signal is silent (peak is 0) or no samples are present.
+    /// </returns>
+    double[] Compute(ReadOnlySpan<byte> pcmData, int channels, int sampleRate, int bitsPerSample, int bucketCount);
+}