feat: add flexible instance configuration support with security improvements

- Add InstanceContext interface for runtime configuration
- Implement dual-mode API client (singleton + instance-specific)
- Add secure SHA-256 hashing for cache keys
- Implement LRU cache with TTL (100 instances, 30min expiry)
- Add comprehensive input validation for URLs and API keys
- Sanitize all logging to prevent API key exposure
- Fix session context cleanup and memory management
- Add comprehensive security and integration tests
- Maintain full backward compatibility for single-player usage

Security improvements based on code review:
- Cache keys are now cryptographically hashed
- API credentials never appear in logs
- Memory-bounded cache prevents resource exhaustion
- Input validation rejects invalid/placeholder values
- Proper cleanup of orphaned session contexts

🤖 Generated with Claude Code

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

package.json

@@ -131,6 +131,7 @@
     "@n8n/n8n-nodes-langchain": "^1.110.0",
     "dotenv": "^16.5.0",
     "express": "^5.1.0",
+    "lru-cache": "^11.2.1",
     "n8n": "^1.111.0",
     "n8n-core": "^1.110.0",
     "n8n-workflow": "^1.108.0",