fix(upload): retire the connection after an early refusal, clean up a failed publish

Two more from review.

Refusing a body before reading it is the point of the streaming cap, but the
unread bytes are still in flight on a connection the response advertises as
keep-alive. Node does not drain them, so the NEXT request on that socket hangs
until it times out — reproducible with an 8MB body against a 1MB cap. The
error response now sets Connection: close whenever the request was not read to
the end.

A failed rename — ENOSPC, a vanished directory — left the fully written
staging file behind. Staging names are per-attempt, so a client that retries
instead of aborting accumulates one per try until the upload expires. The
partial is removed on that path too.

Relates to issue 1403

backend/__tests__/services/chunkedUploadStreaming.test.js

@@ -210,6 +210,21 @@ describe('chunked upload streams the body under a cap (#1403)', () => {
       expect(chunkedUpload.getUploadStatus(uploadId)).toBeNull();
     });
 
+    it('removes the staging file when publishing it fails', async () => {
+      const { uploadId } = await init();
+      const dir = path.join(process.env.STORAGE_PATH, 'chunks', uploadId);
+      // Make the rename fail by putting a directory where the chunk goes.
+      await fs.mkdir(path.join(dir, 'chunk_000000'), { recursive: true });
+
+      await expect(chunkedUpload.uploadChunk(uploadId, 0, countingSource(1024)))
+        .rejects.toThrow();
+
+      // The fully written .part must not survive a failed publish — its name is
+      // per-attempt, so retries would otherwise pile them up until expiry.
+      const leftovers = (await fs.readdir(dir)).filter((f) => f.endsWith('.part'));
+      expect(leftovers).toEqual([]);
+    });
+
     it('does not destroy the request stream when it trips the cap', async () => {
       const { uploadId } = await init();
       const source = countingSource(8 * MB);