Add loudness data to recording metadata

This commit introduces the `Loudness` struct to store replay-gain data
(mean, max, and calculated gain in dB) derived from `ffmpeg`'s
`volumedetect`.

The `RecordingMeta` struct is updated to include an optional `loudness`
field. This allows for consistent playback volume across recordings with
varying microphone levels by applying gain in the browser.

Additionally, new `server/src/loudness.rs` and
`server/src/transcribe/ffmpeg.rs` modules are added. The former defines
constants for replay-gain targets and logic to compute gain, while the
latter handles audio analysis using `ffmpeg` to extract loudness
metrics.

The `#[serde(default)]` attribute on the `loudness` field in
`RecordingMeta` ensures backward compatibility with older metadata files
that do not contain this field. Tests are included to verify round-trip
serialization and parsing of older formats.

server/src/transcribe/worker.rs

@@ -137,6 +137,7 @@ pub async fn run(
         let meta = RecordingMeta {
             transcript,
             duration_seconds,
+            loudness: None,
         };
         if let Err(e) = paths::write_recording_meta_atomic(&audio_path, &meta).await {
             error!(meta = %meta_path.display(), error = %e, "writing recording meta failed");