feat: Add ffmpeg remuxing for faststart

server/src/lib.rs

@@ -3,6 +3,7 @@ pub mod config;
 pub mod error;
 pub mod models;
 pub mod routes;
+pub mod transcribe;
 
 use std::sync::Arc;
 

server/src/transcribe/ffmpeg.rs

@@ -0,0 +1,70 @@
+use std::path::Path;
+use std::process::Stdio;
+
+use tempfile::NamedTempFile;
+use tokio::process::Command;
+use tracing::debug;
+
+#[derive(Debug)]
+pub enum FfmpegError {
+    Spawn(std::io::Error),
+    NonZeroExit { code: Option<i32>, stderr: String },
+    TempFile(std::io::Error),
+}
+
+impl std::fmt::Display for FfmpegError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::Spawn(e) => write!(f, "failed to spawn ffmpeg: {e}"),
+            Self::NonZeroExit { code, stderr } => {
+                write!(f, "ffmpeg exited with {code:?}: {stderr}")
+            }
+            Self::TempFile(e) => write!(f, "failed to create temp file: {e}"),
+        }
+    }
+}
+
+impl std::error::Error for FfmpegError {}
+
+/// Remux an m4a file with `-movflags faststart` so the `moov` atom sits at the
+/// start of the file. Returns a NamedTempFile that is auto-deleted on drop.
+///
+/// Lossless — just a container-level rewrite, typically <1s for dictation-sized files.
+pub async fn remux_faststart(input: &Path) -> Result<NamedTempFile, FfmpegError> {
+    let output = tempfile::Builder::new()
+        .prefix("doctate-remux-")
+        .suffix(".m4a")
+        .tempfile()
+        .map_err(FfmpegError::TempFile)?;
+
+    let output_path = output.path().to_owned();
+
+    let result = Command::new("ffmpeg")
+        .arg("-hide_banner")
+        .arg("-loglevel")
+        .arg("error")
+        .arg("-y")
+        .arg("-i")
+        .arg(input)
+        .arg("-c")
+        .arg("copy")
+        .arg("-movflags")
+        .arg("faststart")
+        .arg(&output_path)
+        .stdin(Stdio::null())
+        .stdout(Stdio::null())
+        .stderr(Stdio::piped())
+        .output()
+        .await
+        .map_err(FfmpegError::Spawn)?;
+
+    if !result.status.success() {
+        return Err(FfmpegError::NonZeroExit {
+            code: result.status.code(),
+            stderr: String::from_utf8_lossy(&result.stderr).into_owned(),
+        });
+    }
+
+    debug!(?output_path, "ffmpeg remux completed");
+    Ok(output)
+}

server/src/transcribe/mod.rs

@@ -0,0 +1 @@
+pub mod ffmpeg;