Fix dashboard timeout by making disk stats non-blocking

virsh guestinfo calls (one per VM) took ~4s on cold cache, exceeding
the dashboard's 3s fetch timeout. Disk stats now refresh in a background
thread and return stale/empty data immediately. Also raised dashboard
fetch timeout to 8s for cold-cache dominfo calls.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app.py

@@ -240,13 +240,8 @@ def get_vm_live_stats():
     return live
 
 
-def get_vm_disk_stats(running_names):
-    """Get filesystem usage for running VMs via virsh guestinfo. Cached for VM_DISK_TTL seconds."""
-    with _vm_disk_lock:
-        now = time.time()
-        if now - _vm_disk_cache["ts"] < VM_DISK_TTL:
-            return _vm_disk_cache["data"]
-
+def _fetch_disk_stats(running_names):
+    """Background worker: fetch filesystem stats for all running VMs."""
     disks = {}
     for name in running_names:
         try:
@@ -256,16 +251,12 @@ def get_vm_disk_stats(running_names):
             )
             if result.returncode != 0:
                 continue
-            # Parse filesystem entries, find root mountpoint
             fs = {}
             for line in result.stdout.split("\n"):
                 if ":" not in line:
                     continue
                 key, val = line.split(":", 1)
-                key = key.strip()
-                val = val.strip()
-                fs[key] = val
-            # Collect all filesystems
+                fs[key.strip()] = val.strip()
             count = int(fs.get("fs.count", 0))
             vm_disks = []
             for i in range(count):
@@ -282,11 +273,23 @@ def get_vm_disk_stats(running_names):
                 disks[name] = vm_disks
         except Exception:
             continue
-
     with _vm_disk_lock:
         _vm_disk_cache["data"] = disks
         _vm_disk_cache["ts"] = time.time()
-    return disks
+        _vm_disk_cache["refreshing"] = False
+
+
+def get_vm_disk_stats(running_names):
+    """Get filesystem usage. Returns cached data immediately; refreshes in background."""
+    with _vm_disk_lock:
+        now = time.time()
+        stale = now - _vm_disk_cache["ts"] >= VM_DISK_TTL
+        refreshing = _vm_disk_cache.get("refreshing", False)
+        if stale and not refreshing:
+            _vm_disk_cache["refreshing"] = True
+            t = threading.Thread(target=_fetch_disk_stats, args=(running_names,), daemon=True)
+            t.start()
+        return _vm_disk_cache["data"]
 
 
 def get_vm_base_info():
@@ -380,7 +383,7 @@ def get_local_stats():
     }
 
 
-def fetch_remote_stats(url, timeout=3):
+def fetch_remote_stats(url, timeout=8):
     try:
         req = urllib.request.Request(url.rstrip("/") + "/api/stats")
         with urllib.request.urlopen(req, timeout=timeout) as resp: