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/RmsLoudnessAlgorithm.cs

@@ -0,0 +1,138 @@
+namespace DeepDrftContent.Processors;
+
+/// <summary>
+/// Loudness via root-mean-square amplitude per time bucket. Decodes signed PCM (8-bit unsigned,
+/// 16/24/32-bit signed little-endian), averages channels to mono, partitions the frames into
+/// equal time slices, takes the RMS of each slice, then peak-normalizes so the loudest bucket is 1.
+/// No external audio dependency — operates directly on the WAV data-chunk bytes.
+/// </summary>
+public class RmsLoudnessAlgorithm : ILoudnessAlgorithm
+{
+    public double[] Compute(ReadOnlySpan<byte> pcmData, int channels, int sampleRate, int bitsPerSample, int bucketCount)
+    {
+        if (bucketCount <= 0)
+        {
+            throw new ArgumentOutOfRangeException(nameof(bucketCount), "Bucket count must be positive.");
+        }
+
+        var result = new double[bucketCount];
+
+        if (channels <= 0)
+        {
+            return result;
+        }
+
+        var bytesPerSample = bitsPerSample / 8;
+        if (bytesPerSample <= 0)
+        {
+            return result;
+        }
+
+        var bytesPerFrame = bytesPerSample * channels;
+        var frameCount = pcmData.Length / bytesPerFrame;
+        if (frameCount == 0)
+        {
+            return result;
+        }
+
+        // Sum of squared mono amplitudes and the frame count, per bucket. A frame's bucket is
+        // determined by its position in the timeline so buckets are equal-duration slices.
+        var sumSquares = new double[bucketCount];
+        var counts = new long[bucketCount];
+
+        for (var frame = 0; frame < frameCount; frame++)
+        {
+            var frameStart = frame * bytesPerFrame;
+
+            double channelSum = 0;
+            for (var ch = 0; ch < channels; ch++)
+            {
+                var sampleStart = frameStart + ch * bytesPerSample;
+                channelSum += ReadSampleNormalized(pcmData, sampleStart, bitsPerSample);
+            }
+
+            var mono = channelSum / channels;
+
+            // long math avoids overflow on large files before the divide back into bucket index.
+            var bucket = (int)((long)frame * bucketCount / frameCount);
+            if (bucket >= bucketCount)
+            {
+                bucket = bucketCount - 1;
+            }
+
+            sumSquares[bucket] += mono * mono;
+            counts[bucket]++;
+        }
+
+        var peak = 0.0;
+        for (var i = 0; i < bucketCount; i++)
+        {
+            if (counts[i] > 0)
+            {
+                result[i] = Math.Sqrt(sumSquares[i] / counts[i]);
+                if (result[i] > peak)
+                {
+                    peak = result[i];
+                }
+            }
+        }
+
+        if (peak <= 0)
+        {
+            // Silence — return all zeros (Array is already zero-initialized).
+            Array.Clear(result);
+            return result;
+        }
+
+        for (var i = 0; i < bucketCount; i++)
+        {
+            result[i] /= peak;
+        }
+
+        return result;
+    }
+
+    /// <summary>
+    /// Decodes one PCM sample at <paramref name="offset"/> to a normalized amplitude in [-1, 1].
+    /// 8-bit is unsigned (0..255, centered at 128); 16/24/32-bit are signed little-endian.
+    /// </summary>
+    private static double ReadSampleNormalized(ReadOnlySpan<byte> data, int offset, int bitsPerSample)
+    {
+        switch (bitsPerSample)
+        {
+            case 8:
+                // Unsigned, midpoint 128.
+                return (data[offset] - 128) / 128.0;
+
+            case 16:
+            {
+                short sample = (short)(data[offset] | (data[offset + 1] << 8));
+                return sample / 32768.0;
+            }
+
+            case 24:
+            {
+                // Sign-extend the 24-bit little-endian value into an int.
+                int raw = data[offset] | (data[offset + 1] << 8) | (data[offset + 2] << 16);
+                if ((raw & 0x800000) != 0)
+                {
+                    raw |= unchecked((int)0xFF000000);
+                }
+                return raw / 8388608.0;
+            }
+
+            case 32:
+            {
+                int sample = data[offset]
+                             | (data[offset + 1] << 8)
+                             | (data[offset + 2] << 16)
+                             | (data[offset + 3] << 24);
+                return sample / 2147483648.0;
+            }
+
+            default:
+                throw new ArgumentOutOfRangeException(
+                    nameof(bitsPerSample), bitsPerSample, "Unsupported PCM bit depth.");
+        }
+    }
+}