fix: resolve document upload hang by fixing stream pipeline (refs #33)

The upload was hanging silently because breaking early from a
`for await` loop on a Node.js stream corrupts the stream's internal
state. The remaining stream could not be used afterward.

Changes:
- Collect ALL chunks from the file stream before processing
- Use subarray() for file type detection header (first 4100 bytes)
- Create single readable stream from complete buffer for storage
- Remove broken headerStream + remainingStream piping logic

This fixes the root cause where uploads would hang after logging
"Document upload requested" without ever completing or erroring.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/features/documents/api/documents.controller.ts

@@ -272,20 +272,15 @@ export class DocumentsController {
       });
     }
 
-    // Read first 4100 bytes to detect file type via magic bytes
+    // Collect ALL file chunks first (breaking early from async iterator corrupts stream state)
     const chunks: Buffer[] = [];
-    let totalBytes = 0;
-    const targetBytes = 4100;
-
     for await (const chunk of mp.file) {
       chunks.push(chunk);
-      totalBytes += chunk.length;
-      if (totalBytes >= targetBytes) {
-        break;
-      }
     }
+    const fullBuffer = Buffer.concat(chunks);
 
-    const headerBuffer = Buffer.concat(chunks);
+    // Use first 4100 bytes for file type detection via magic bytes
+    const headerBuffer = fullBuffer.subarray(0, Math.min(4100, fullBuffer.length));
 
     // Validate actual file content using magic bytes
     const detectedType = await FileType.fromBuffer(headerBuffer);
@@ -341,15 +336,9 @@ export class DocumentsController {
 
     const counter = new CountingStream();
 
-    // Create a new readable stream from the header buffer + remaining file chunks
-    const headerStream = Readable.from([headerBuffer]);
-    const remainingStream = mp.file;
-
-    // Pipe header first, then remaining content through counter
-    headerStream.pipe(counter, { end: false });
-    headerStream.on('end', () => {
-      remainingStream.pipe(counter);
-    });
+    // Create readable stream from the complete buffer and pipe through counter
+    const fileStream = Readable.from([fullBuffer]);
+    fileStream.pipe(counter);
 
     const storage = getStorageService();
     const bucket = 'documents';