fix: stream cover uploads with a per-chunk size cap

The cover multipart path called Field::bytes(), buffering the entire field
into memory before parse_image checked max_file_bytes — an oversized cover
was fully allocated before rejection. Add read_capped/push_capped, which
reject with 413 as soon as the running total exceeds the cap (the offending
chunk is never copied in), and route both cover handlers and stage_image_part
through it.

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

backend/src/api/mangas.rs

@@ -239,7 +239,8 @@ async fn create(
                 metadata = Some(parse_metadata_json(&bytes)?);
             }
             Some("cover") => {
-                let bytes = read_field_bytes(field).await?.to_vec();
+                let bytes =
+                    crate::upload::read_capped(field, state.upload.max_file_bytes, "cover").await?;
                 cover = Some(parse_image(bytes, state.upload.max_file_bytes, "cover")?);
             }
             _ => continue,
@@ -387,7 +388,8 @@ async fn put_cover(
     let mut cover: Option<UploadedImage> = None;
     while let Some(field) = next_field(&mut multipart).await? {
         if field.name() == Some("cover") {
-            let bytes = read_field_bytes(field).await?.to_vec();
+            let bytes =
+                crate::upload::read_capped(field, state.upload.max_file_bytes, "cover").await?;
             cover = Some(parse_image(bytes, state.upload.max_file_bytes, "cover")?);
         }
     }

backend/src/upload/mod.rs

@@ -45,21 +45,13 @@ pub const STAGING_PREFIX: &str = "staging";
 /// read-all-then-persist path.
 pub async fn stage_image_part(
     storage: &dyn Storage,
-    mut field: Field<'_>,
+    field: Field<'_>,
     upload_id: Uuid,
     seq: usize,
     max_size: usize,
     field_name: &str,
 ) -> AppResult<StagedImage> {
-    let mut bytes: Vec<u8> = Vec::new();
-    while let Some(chunk) = field.chunk().await.map_err(map_multipart_error)? {
-        if bytes.len().saturating_add(chunk.len()) > max_size {
-            return Err(AppError::PayloadTooLarge(format!(
-                "{field_name} exceeds {max_size}-byte cap"
-            )));
-        }
-        bytes.extend_from_slice(&chunk);
-    }
+    let bytes = read_capped(field, max_size, field_name).await?;
     // Reuse the shared sniff + whitelist check; it re-verifies the size cap
     // (already enforced above) and derives mime/ext from magic bytes.
     let img = parse_image(bytes, max_size, field_name)?;
@@ -73,6 +65,46 @@ pub async fn stage_image_part(
     })
 }
 
+/// Read one multipart part fully into memory, enforcing the per-file byte cap
+/// as chunks arrive so an oversized part is rejected (413) **without being fully
+/// buffered**. Use for parts whose bytes the caller needs in hand — e.g. the
+/// cover image, which is written to a manga-scoped key. Page parts should prefer
+/// [`stage_image_part`], which streams straight to storage.
+///
+/// Replaces the `Field::bytes()` path, which buffered the entire field before
+/// any size check ran, letting an attacker allocate an arbitrarily large body
+/// before the cap kicked in.
+pub async fn read_capped(
+    mut field: Field<'_>,
+    max_size: usize,
+    field_name: &str,
+) -> AppResult<Vec<u8>> {
+    let mut bytes: Vec<u8> = Vec::new();
+    while let Some(chunk) = field.chunk().await.map_err(map_multipart_error)? {
+        push_capped(&mut bytes, &chunk, max_size, field_name)?;
+    }
+    Ok(bytes)
+}
+
+/// Append `chunk` to `buf`, rejecting with 413 as soon as the running total
+/// would exceed `max_size` — so the oversized chunk is never copied in. Split
+/// out from the read loop so the cap logic is unit-testable without a live
+/// multipart field.
+fn push_capped(
+    buf: &mut Vec<u8>,
+    chunk: &[u8],
+    max_size: usize,
+    field_name: &str,
+) -> AppResult<()> {
+    if buf.len().saturating_add(chunk.len()) > max_size {
+        return Err(AppError::PayloadTooLarge(format!(
+            "{field_name} exceeds {max_size}-byte cap"
+        )));
+    }
+    buf.extend_from_slice(chunk);
+    Ok(())
+}
+
 pub fn parse_image(bytes: Vec<u8>, max_size: usize, field_name: &str) -> AppResult<UploadedImage> {
     if bytes.len() > max_size {
         return Err(AppError::PayloadTooLarge(format!(
@@ -172,4 +204,30 @@ mod tests {
         assert!(matches!(err, AppError::PayloadTooLarge(_)));
         assert_eq!(err.code(), "payload_too_large");
     }
+
+    #[test]
+    fn push_capped_accumulates_under_cap() {
+        let mut buf = Vec::new();
+        push_capped(&mut buf, b"hello ", 100, "cover").unwrap();
+        push_capped(&mut buf, b"world", 100, "cover").unwrap();
+        assert_eq!(buf, b"hello world");
+    }
+
+    #[test]
+    fn push_capped_rejects_before_copying_oversized_chunk() {
+        // The whole point: the offending chunk must NOT be appended — an
+        // oversized part is rejected without buffering it.
+        let mut buf = vec![0u8; 90];
+        let err = push_capped(&mut buf, &[0u8; 20], 100, "cover").unwrap_err();
+        assert!(matches!(err, AppError::PayloadTooLarge(_)));
+        assert_eq!(err.code(), "payload_too_large");
+        assert_eq!(buf.len(), 90, "buffer must not grow past the cap");
+    }
+
+    #[test]
+    fn push_capped_allows_exactly_at_cap() {
+        let mut buf = vec![0u8; 90];
+        push_capped(&mut buf, &[0u8; 10], 100, "cover").unwrap();
+        assert_eq!(buf.len(), 100);
+    }
 }