perf(transcode): eliminate double buffer copy by accepting Bytes instead of &[u8]

Change ImageTranscodePort::get_transcoded signature from &[u8] to Bytes.
- Rayon closure now receives Bytes::clone() (O(1) ref-count) instead of .to_vec() (~5 MB copy)
- Fallback path returns owned Bytes directly instead of Bytes::from(to_vec()) (~5 MB copy)
- Caller passes content.clone() (O(1)) instead of implicit deref

Saves ~10 MB of allocation per transcode call on a 5 MB image.

src/application/ports/transcode_ports.rs

@@ -90,7 +90,7 @@ pub trait ImageTranscodePort: Send + Sync + 'static {
     async fn get_transcoded(
         &self,
         file_id: &str,
-        original_content: &[u8],
+        original_content: Bytes,
         original_mime: &str,
         target_format: OutputFormat,
     ) -> Result<(Bytes, String, bool), DomainError>;

src/application/services/file_retrieval_service.rs

@@ -88,7 +88,7 @@ impl FileRetrievalService {
             return None;
         }
         let format = OutputFormat::WebP;
-        match transcode.get_transcoded(id, content, mime, format).await {
+        match transcode.get_transcoded(id, content.clone(), mime, format).await {
             Ok((transcoded, webp_mime, true)) => {
                 debug!(
                     "🖼️ WebP transcode: {} -> {} bytes ({:.0}% smaller)",

src/infrastructure/services/image_transcode_service.rs

@@ -192,10 +192,13 @@ impl ImageTranscodeService {
 
     /// Get transcoded version of an image.
     /// Returns `(content, mime_type, was_transcoded)`.
+    ///
+    /// Accepts `Bytes` (ref-counted) so callers avoid copying the buffer.
+    /// Cloning `Bytes` is O(1) — only an atomic increment.
     pub async fn get_transcoded(
         &self,
         file_id: &str,
-        original_content: &[u8],
+        original_content: Bytes,
         original_mime: &str,
         target_format: OutputFormat,
     ) -> Result<(Bytes, String, bool), String> {
@@ -228,13 +231,13 @@ impl ImageTranscodeService {
         }
 
         // ── 3. Transcode on dedicated rayon pool (never blocks Tokio) ──
-        let content_owned = original_content.to_vec();
+        let content_for_rayon = original_content.clone(); // O(1) ref-count bump
         let mime_owned = original_mime.to_string();
 
         let (tx, rx) = tokio::sync::oneshot::channel();
 
         transcode_pool().spawn(move || {
-            let result = transcode_image_blocking(&content_owned, &mime_owned, target_format);
+            let result = transcode_image_blocking(&content_for_rayon, &mime_owned, target_format);
             let _ = tx.send(result);
         });
 
@@ -256,7 +259,7 @@ impl ImageTranscodeService {
                 transcoded_size
             );
             return Ok((
-                Bytes::from(original_content.to_vec()),
+                original_content,
                 original_mime.to_string(),
                 false,
             ));
@@ -394,7 +397,7 @@ impl ImageTranscodePort for ImageTranscodeService {
     async fn get_transcoded(
         &self,
         file_id: &str,
-        original_content: &[u8],
+        original_content: Bytes,
         original_mime: &str,
         target_format: PortOutputFormat,
     ) -> Result<(Bytes, String, bool), DomainError> {