fix: respect NODE_DB_PATH environment variable in Docker (fixes #40)

- Fixed docker-entrypoint.sh to use NODE_DB_PATH instead of hardcoded paths
- Added log_message() helper to prevent stdio mode output corruption
- Fixed directory creation race condition with proper ownership
- Added path validation to ensure NODE_DB_PATH ends with .db
- Updated rebuild.ts to respect NODE_DB_PATH environment variable
- Added comprehensive documentation for custom database paths
- Bumped version to 2.7.16

The bug was caused by hardcoded /app/data/nodes.db paths in the Docker
entrypoint script that ignored the NODE_DB_PATH environment variable.

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

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "n8n-mcp",
-  "version": "2.7.15",
+  "version": "2.7.16",
   "description": "Integration between n8n workflow automation and Model Context Protocol (MCP)",
   "main": "dist/index.js",
   "bin": {