feat: background device status check every 240s instead of on page load

- Add online, leds, version, last_check columns to wled_devices table
- Background job checks all WLED devices every 4 minutes
- Initial check on startup
- Devices page reads status from DB (instant page load)
- 'Alle jetzt pruefen' button triggers manual check

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

database.py

@@ -32,7 +32,11 @@ def init_db():
             id INTEGER PRIMARY KEY AUTOINCREMENT,
             hostname TEXT NOT NULL UNIQUE,
             name TEXT DEFAULT '',
-            segment_count INTEGER NOT NULL DEFAULT 2
+            segment_count INTEGER NOT NULL DEFAULT 2,
+            online INTEGER DEFAULT 0,
+            leds INTEGER DEFAULT 0,
+            version TEXT DEFAULT '',
+            last_check TIMESTAMP
         );
 
         CREATE TABLE IF NOT EXISTS assignments (
@@ -71,6 +75,24 @@ def init_db():
         );
     """)
 
+    # Migrate: add columns if missing (for existing databases)
+    try:
+        conn.execute("ALTER TABLE wled_devices ADD COLUMN online INTEGER DEFAULT 0")
+    except Exception:
+        pass
+    try:
+        conn.execute("ALTER TABLE wled_devices ADD COLUMN leds INTEGER DEFAULT 0")
+    except Exception:
+        pass
+    try:
+        conn.execute("ALTER TABLE wled_devices ADD COLUMN version TEXT DEFAULT ''")
+    except Exception:
+        pass
+    try:
+        conn.execute("ALTER TABLE wled_devices ADD COLUMN last_check TIMESTAMP")
+    except Exception:
+        pass
+
     defaults = {
         "azure_client_id": "",
         "azure_client_secret": "",