Address code review: compute stderr_text only when chunks are present

app/sovran_systemsos_web/server.py

@@ -3762,8 +3762,11 @@ async def _monitor_backup_subprocess(proc: asyncio.subprocess.Process) -> None:
     if status in {"SUCCESS", "FAILED"}:
         return
 
-    stderr_text = b"".join(stderr_chunks).decode("utf-8", errors="replace").strip()
-    detail = f" — stderr: {stderr_text}" if stderr_text else ""
+    detail = ""
+    if stderr_chunks:
+        stderr_text = b"".join(stderr_chunks).decode("utf-8", errors="replace").strip()
+        if stderr_text:
+            detail = f" — stderr: {stderr_text}"
     msg = f"[{time.strftime('%Y-%m-%d %H:%M:%S')}] ERROR: Backup subprocess exited unexpectedly (code {rc}).{detail}"
     await loop.run_in_executor(None, _append_backup_log, msg)
     await loop.run_in_executor(None, _write_backup_status, "FAILED")

app/tests/test_manual_backup_workflow.py

@@ -108,8 +108,10 @@ class ManualBackupWorkflowTests(unittest.TestCase):
         the FAILED log entry so the UI shows what went wrong (e.g. 'bash: not
         found') rather than only the raw exit code."""
         source = SERVER_FILE.read_text()
+        self.assertIn("stderr_chunks", source)
         self.assertIn("stderr_text", source)
-        self.assertIn('if stderr_text else ""', source)
+        # stderr detail is conditionally appended only when non-empty
+        self.assertIn('detail = f" — stderr: {stderr_text}"', source)
 
     def test_exit_code_127_subprocess_stderr_drain(self):
         """Behavioral regression: a subprocess that exits 127 must have its