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/tests/api_uploads.rs

@@ -172,6 +172,13 @@ async fn files_endpoint_streams_in_multiple_frames(pool: PgPool) {
         resp.headers().get("x-content-type-options").unwrap(),
         "nosniff"
     );
+    // Blobs are content-addressed by unguessable, immutable keys, so they're
+    // safe to cache forever — this is what makes the reader's page + next-
+    // chapter preloading actually hit cache instead of re-downloading.
+    assert_eq!(
+        resp.headers().get(header::CACHE_CONTROL).unwrap(),
+        "public, max-age=31536000, immutable"
+    );
 
     let mut body = resp.into_body();
     let mut frames = 0usize;