feat: backup system for portal DB and WLED configs

Portal DB:
- Download current DB button
- Manual backup creation
- Auto-backup daily at 02:00 (keeps last 14)
- Download/delete individual backups

WLED Config:
- Backup all devices at once
- Backup individual device
- Download config as JSON
- Restore config to any device
- Delete old backups

New sidebar menu item 'Backup' with dedicated page.

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

wled_client.py

@@ -45,3 +45,27 @@ def get_wled_info(hostname):
         return {"online": False}
     except requests.exceptions.RequestException:
         return {"online": False}
+
+
+def get_wled_config(hostname):
+    """Download full WLED config as JSON."""
+    try:
+        r = requests.get(f"http://{hostname}/cfg.json", timeout=TIMEOUT)
+        if r.status_code == 200:
+            return r.json()
+        return None
+    except requests.exceptions.RequestException:
+        return None
+
+
+def restore_wled_config(hostname, config_json):
+    """Push config JSON to WLED device."""
+    try:
+        r = requests.post(
+            f"http://{hostname}/json/cfg",
+            json=config_json,
+            timeout=TIMEOUT,
+        )
+        return r.status_code == 200, f"{r.status_code} {r.reason}"
+    except requests.exceptions.RequestException as e:
+        return False, str(e)