fix: don't mark private-mode blobs as publicly cacheable

The immutable Cache-Control added for the reader preload work was
`public` unconditionally. Under PRIVATE_MODE, /files is auth-gated, so a
shared cache/CDN would store the blob and serve it to unauthenticated
clients, defeating the gate. Emit `private, ...` when private_mode is on,
`public, ...` otherwise.

Found in the security audit; regression from 0.124.3. Bump to 0.124.9.

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

backend/src/api/files.rs

@@ -55,9 +55,19 @@ async fn serve(State(state): State<AppState>, Path(key): Path<String>) -> AppRes
         // 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).
+        //
+        // BUT under PRIVATE_MODE these blobs are auth-gated (see
+        // `private_mode_guard`), so they must NOT be marked `public`: a shared
+        // cache / CDN in front of the app would store the object and then serve
+        // it to unauthenticated clients, defeating the gate. Use `private` so
+        // only the requesting user's browser caches it.
         (
             header::CACHE_CONTROL,
-            "public, max-age=31536000, immutable".to_string(),
+            if state.auth.private_mode {
+                "private, max-age=31536000, immutable".to_string()
+            } else {
+                "public, max-age=31536000, immutable".to_string()
+            },
         ),
     ];
     Ok((headers, Body::from_stream(file.stream)).into_response())