chore: upgrade Docker base images to Node.js 22 LTS Alpine (v2.7.22)

- Updated all Dockerfiles from node:20-alpine to node:22-alpine
- Addresses known vulnerabilities in older Alpine images
- Provides better long-term support with Node.js 22 LTS (until April 2027)
- Updated documentation to reflect new base image version
- Tested and verified compatibility with all dependencies

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

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

Dockerfile.railway

@@ -2,7 +2,7 @@
 # Railway-compatible Dockerfile for n8n-mcp
 
 # --- Stage 1: Builder ---
-FROM node:20-alpine AS builder
+FROM node:22-alpine AS builder
 WORKDIR /app
 
 # Install system dependencies for native modules
@@ -22,7 +22,7 @@ COPY src ./src
 RUN npm run build
 
 # --- Stage 2: Runtime ---
-FROM node:20-alpine AS runtime
+FROM node:22-alpine AS runtime
 WORKDIR /app
 
 # Install system dependencies