docs: add complete guide, env files, fix docker-compose

- Add GUIDE.md: complete user documentation covering installation,
  Docker deployment, AI providers, MCP server, N8N integration,
  email config, admin panel, env var reference, troubleshooting
- Add mcp-server/.env.example with all MCP-specific variables
- Update .env.docker.example with all 42 environment variables
- Fix docker-compose.yml: parameterize PostgreSQL credentials,
  add missing env vars (CUSTOM_OPENAI, AI_PROVIDER_CHAT,
  ALLOW_REGISTRATION, RESEND_API_KEY)
- Track memento-note/.env.example

mcp-server/.env.example

@@ -0,0 +1,50 @@
+# =============================================================================
+# MCP Server - Environment Variables
+# =============================================================================
+# Copy this file to .env and fill in the values.
+# cp .env.example .env
+
+# =============================================================================
+# DATABASE (REQUIRED)
+# =============================================================================
+# Local development (if sharing SQLite with memento-note):
+# DATABASE_URL="file:../memento-note/prisma/dev.db"
+# PostgreSQL:
+DATABASE_URL="postgresql://memento:memento@localhost:5432/memento"
+
+# =============================================================================
+# MCP SERVER MODE
+# =============================================================================
+# 'stdio' = Standard I/O (Claude Desktop, Cline, etc.)
+# 'sse'   = Server-Sent Events / HTTP (N8N, remote access, port 3001)
+MCP_MODE="stdio"
+
+# =============================================================================
+# SSE MODE SETTINGS (only if MCP_MODE="sse")
+# =============================================================================
+# Port for HTTP/SSE mode
+# PORT=3001
+
+# Request timeout in milliseconds (default: 30000)
+# MCP_REQUEST_TIMEOUT=30000
+
+# Log level: debug, info, warn, error (default: info)
+# MCP_LOG_LEVEL=info
+
+# =============================================================================
+# AUTHENTICATION (SSE mode only)
+# =============================================================================
+# Enable authentication (default: false in dev mode)
+# MCP_REQUIRE_AUTH=false
+
+# Static API key for authentication (if MCP_REQUIRE_AUTH=true)
+# MCP_API_KEY="your-secret-api-key"
+
+# =============================================================================
+# APPLICATION
+# =============================================================================
+# Base URL of the Memento web app (for generating links in responses)
+# APP_BASE_URL="http://localhost:3000"
+
+# Filter data to a specific user ID (optional, null = all users)
+# USER_ID=""