chore: update n8n to 2.1.5 and bump version to 2.31.6 (#521)

- Updated n8n from 2.1.4 to 2.1.5
- Updated n8n-core from 2.1.3 to 2.1.4
- Updated @n8n/n8n-nodes-langchain from 2.1.3 to 2.1.4
- Rebuilt node database with 540 nodes (434 from n8n-nodes-base, 106 from @n8n/n8n-nodes-langchain)
- Updated README badge with new n8n version
- Updated CHANGELOG with dependency changes

Conceived by Romuald Członkowski - https://www.aiadvisors.pl/en

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Romuald Członkowski <romualdczlonkowski@MacBook-Pro-Romuald.local>
Co-authored-by: Claude <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "n8n-mcp",
-  "version": "2.31.5",
+  "version": "2.31.6",
   "description": "Integration between n8n workflow automation and Model Context Protocol (MCP)",
   "main": "dist/index.js",
   "types": "dist/index.d.ts",
@@ -141,15 +141,15 @@
   },
   "dependencies": {
     "@modelcontextprotocol/sdk": "1.20.1",
-    "@n8n/n8n-nodes-langchain": "^2.1.3",
+    "@n8n/n8n-nodes-langchain": "^2.1.4",
     "@supabase/supabase-js": "^2.57.4",
     "dotenv": "^16.5.0",
     "express": "^5.1.0",
     "express-rate-limit": "^7.1.5",
     "form-data": "^4.0.5",
     "lru-cache": "^11.2.1",
-    "n8n": "^2.1.4",
-    "n8n-core": "^2.1.3",
+    "n8n": "^2.1.5",
+    "n8n-core": "^2.1.4",
     "n8n-workflow": "^2.1.1",
     "openai": "^4.77.0",
     "sql.js": "^1.13.0",