fix: cap the multipart metadata part instead of buffering it unbounded

The `metadata` JSON part in the manga-create and chapter-upload handlers was
read with `Field::bytes()`, buffering the whole part before any check — a client
could send a multi-hundred-MB metadata blob (up to the 200 MiB request limit) as
one allocation. Image parts were already streamed with a cap; metadata was the
last unbounded read.

Cap it at a shared upload::MAX_METADATA_BYTES (64 KiB) via the read_capped
streaming loop (413 on overflow), and remove the now-unused read_field_bytes
helper so the unbounded path can't return.

Tests: manga + chapter oversized-metadata parts are rejected with 413.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/tests/api_chapters.rs

@@ -358,3 +358,27 @@ async fn non_owner_can_upload_chapter(pool: PgPool) {
     assert_eq!(body["title"], "Contributed");
     assert_eq!(body["page_count"], 1);
 }
+
+#[sqlx::test(migrations = "./migrations")]
+async fn chapter_upload_rejects_oversized_metadata_part(pool: PgPool) {
+    // The chapter `metadata` JSON part is capped as bytes arrive, just like the
+    // manga one, so a client can't buffer a huge JSON blob in memory. A ~200 KiB
+    // title blows the metadata cap before any page is staged.
+    let h = common::harness(pool);
+    let (_, cookie) = common::register_user(&h.app).await;
+    let manga_id = seed_manga(&h, &cookie, "Berserk").await;
+
+    let huge = "A".repeat(200 * 1024);
+    let resp = h
+        .app
+        .oneshot(common::post_multipart_with_cookie(
+            &format!("/api/v1/mangas/{manga_id}/chapters"),
+            MultipartBuilder::new()
+                .add_json("metadata", json!({ "number": 1, "title": huge }))
+                .add_file("page", "1.png", "image/png", &common::fake_png_bytes()),
+            &cookie,
+        ))
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), StatusCode::PAYLOAD_TOO_LARGE);
+}

backend/tests/api_mangas_metadata.rs

@@ -927,3 +927,25 @@ async fn bearer_authed_admin_cannot_edit_null_uploader(pool: PgPool) {
     );
 }
 
+
+#[sqlx::test(migrations = "./migrations")]
+async fn create_rejects_oversized_metadata_part(pool: PgPool) {
+    // The metadata JSON part is capped well below the 200 MiB request limit so a
+    // client can't force the server to buffer a huge JSON blob in memory before
+    // any validation runs. A ~200 KiB description blows the metadata cap.
+    let h = common::harness(pool);
+    let (_, cookie) = common::register_user(&h.app).await;
+
+    let huge = "A".repeat(200 * 1024);
+    let resp = h
+        .app
+        .oneshot(common::post_multipart_with_cookie(
+            "/api/v1/mangas",
+            MultipartBuilder::new()
+                .add_json("metadata", json!({ "title": "Big", "description": huge })),
+            &cookie,
+        ))
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), StatusCode::PAYLOAD_TOO_LARGE);
+}