ConnLogs: return object with _diag for troubleshooting + frontend compat

API now returns:
{
  "logs": [...],    // ConnectionLog array (same fields as before)
  "_diag": [...]    // status.log paths/existence/parse status per instance
}

Frontend updated to handle both array (legacy) and object format.

This makes it easy to diagnose:
- Which status.log paths the backend looked at
- Whether files exist and their sizes
- How many entries were parsed
- Any parse errors

User just needs to: git pull && rebuild && restart on production.

frontend/src/views/Logs.vue

@@ -33,7 +33,11 @@ const list = ref([])
 const instances = ref([])
 const instanceId = ref('')
 
-async function load() { list.value = await Logs.conns(instanceId.value) }
+async function load() {
+  const r = await Logs.conns(instanceId.value)
+  // 后端可能返回数组（旧）或 {logs:[...], _diag:[...]}（新）
+  list.value = Array.isArray(r) ? r : (r.logs || [])
+}
 function fmt(n) {
   if (!n) return '0'
   const u = ['B','KB','MB','GB','TB']; let i=0,v=n