feat(audio): add MP3 and FLAC upload support via format-routed processors

AudioProcessorRouter dispatches by extension; vault stores original bytes with correct MIME type.

DeepDrftContent/Processors/FlacAudioProcessor.cs

@@ -0,0 +1,104 @@
+using DeepDrftContent.FileDatabase.Models;
+
+namespace DeepDrftContent.Processors;
+
+/// <summary>
+/// Extracts metadata from a FLAC file and wraps its <b>unmodified</b> bytes in an
+/// <see cref="AudioBinary"/> tagged <c>.flac</c>. No transcoding — the vault stores the original
+/// stream; duration and average bitrate come from the mandatory STREAMINFO metadata block.
+/// </summary>
+public class FlacAudioProcessor
+{
+    private const double FallbackDuration = 180.0;
+    private const int FallbackBitrate = 1411;
+
+    public async Task<AudioBinary?> ProcessFlacFileAsync(string filePath)
+    {
+        if (!File.Exists(filePath))
+        {
+            throw new FileNotFoundException($"FLAC file not found: {filePath}");
+        }
+
+        if (!Path.GetExtension(filePath).Equals(".flac", StringComparison.OrdinalIgnoreCase))
+        {
+            throw new ArgumentException("File must be a FLAC file", nameof(filePath));
+        }
+
+        var buffer = await File.ReadAllBytesAsync(filePath);
+        var meta = ExtractFlacMetadata(buffer);
+
+        var parameters = new AudioBinaryParams(
+            Buffer: buffer,
+            Size: buffer.Length,
+            Extension: ".flac",
+            Duration: meta.Duration,
+            Bitrate: meta.Bitrate);
+
+        return new AudioBinary(parameters);
+    }
+
+    /// <summary>
+    /// Validates the <c>fLaC</c> magic and the leading STREAMINFO block, then computes duration from
+    /// total-samples / sample-rate and average bitrate from file size. On any parse failure, logs a
+    /// warning and returns synthetic defaults — never throws.
+    /// </summary>
+    private static FlacMetadata ExtractFlacMetadata(byte[] buffer)
+    {
+        try
+        {
+            // Magic (4) + metadata block header (4) + STREAMINFO data (34) = 42 bytes minimum.
+            if (buffer.Length < 42)
+            {
+                throw new InvalidDataException("File too short for FLAC STREAMINFO");
+            }
+
+            if (buffer[0] != 'f' || buffer[1] != 'L' || buffer[2] != 'a' || buffer[3] != 'C')
+            {
+                throw new InvalidDataException("Invalid fLaC magic");
+            }
+
+            // Metadata block header at offset 4: bits 6-0 of byte 0 are the block type (0 = STREAMINFO).
+            var blockType = buffer[4] & 0x7F;
+            if (blockType != 0)
+            {
+                throw new InvalidDataException($"First metadata block is not STREAMINFO (type {blockType})");
+            }
+
+            // STREAMINFO data begins at offset 8. Layout (bit-packed, big-endian):
+            //   bytes 10-12 + top nibble of 13: sample rate (20 bits)
+            //   bits 3-1 of byte 12: channels - 1
+            //   bit 0 of byte 12 + top 4 bits of byte 13: bits per sample - 1
+            //   low nibble of byte 13 + bytes 14-17: total samples (36 bits)
+            var d = 8;
+            var sampleRate = (buffer[d + 10] << 12) | (buffer[d + 11] << 4) | (buffer[d + 12] >> 4);
+            var totalSamples = ((long)(buffer[d + 13] & 0x0F) << 32)
+                | ((long)buffer[d + 14] << 24)
+                | ((long)buffer[d + 15] << 16)
+                | ((long)buffer[d + 16] << 8)
+                | buffer[d + 17];
+
+            if (sampleRate <= 0)
+            {
+                throw new InvalidDataException("Invalid FLAC sample rate");
+            }
+
+            var duration = (double)totalSamples / sampleRate;
+            var bitrate = duration > 0
+                ? (int)(buffer.LongLength * 8L / (duration * 1000))
+                : FallbackBitrate;
+
+            return new FlacMetadata { Duration = duration, Bitrate = bitrate };
+        }
+        catch (Exception ex)
+        {
+            Console.WriteLine($"Warning: FLAC parsing failed, using defaults: {ex.Message}");
+            return new FlacMetadata { Duration = FallbackDuration, Bitrate = FallbackBitrate };
+        }
+    }
+
+    private sealed class FlacMetadata
+    {
+        public double Duration { get; init; }
+        public int Bitrate { get; init; }
+    }
+}