feat: add get_vlans, get_gateway_interfaces tools and billing config fallback

- Add get_vlans tool to filter interfaces by type=vlan
- Add get_gateway_interfaces tool to identify gateway interfaces from routes
- Add fallback to billing-mcp/config.json when router not in devices.json
- Update .gitignore to exclude venv, pycache, snapshots

devices.json

@@ -0,0 +1,37 @@
+[
+  {
+    "name": "Router-Utama",
+    "host": "10.8.0.17",
+    "port": 80,
+    "user": "admin11",
+    "pass": "Dua75316__"
+  },
+  {
+    "name": "router-dimensi-dell",
+    "host": "103.138.63.178",
+    "port": 80,
+    "user": "chatbot",
+    "pass": "K0s0ng11@2026"
+  },
+  {
+    "name": "ccr1036",
+    "host": "103.138.63.184",
+    "port": 80,
+    "user": "chatbot",
+    "pass": "K0s0ng11@2026"
+  },
+  {
+    "name": "router-smc-core",
+    "host": "103.138.63.183",
+    "port": 81,
+    "user": "chatbot",
+    "pass": "K0s0ng11@2026"
+  },
+  {
+    "name": "router-smc-lb",
+    "host": "103.138.63.183",
+    "port": 80,
+    "user": "chatbot",
+    "pass": "K0s0ng11@2026"
+  }
+]