fix: cache immutable blobs served from /files

Page and cover blobs are content-addressed by unguessable, immutable keys,
but /files sent no Cache-Control, so the browser refetched on every reader
open — defeating the whole-chapter and next-chapter preloading (and re-
proxying every byte through the node server in prod). Send
`Cache-Control: public, max-age=31536000, immutable`.

Bump to 0.124.3.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/src/api/files.rs

@@ -49,6 +49,16 @@ async fn serve(State(state): State<AppState>, Path(key): Path<String>) -> AppRes
             HeaderName::from_static("x-content-type-options"),
             "nosniff".to_string(),
         ),
+        // Blobs are content-addressed by unguessable, immutable keys (a
+        // re-upload mints new UUIDs), so a fetched page/cover never changes.
+        // Cache it for a year and mark it `immutable` so browsers skip
+        // revalidation entirely — this is what lets the reader's page and
+        // next-chapter preloading hit cache instead of re-downloading (and
+        // re-proxying every byte through the SvelteKit node server in prod).
+        (
+            header::CACHE_CONTROL,
+            "public, max-age=31536000, immutable".to_string(),
+        ),
     ];
     Ok((headers, Body::from_stream(file.stream)).into_response())
 }