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

memento-note/.env.example

@@ -0,0 +1,64 @@
+# =============================================================================
+# Memento Note - Environment Variables
+# Copy this file to .env and fill in the values
+# =============================================================================
+
+# -----------------------------------------------------------------------------
+# Core (required)
+# -----------------------------------------------------------------------------
+DATABASE_URL="postgresql://user:password@localhost:5432/memento"
+NEXTAUTH_SECRET="generate-with-openssl-rand-base64-32"
+NEXTAUTH_URL="http://localhost:3000"
+
+# -----------------------------------------------------------------------------
+# Registration
+# -----------------------------------------------------------------------------
+# Set to "false" to disable public registration (default: true)
+# ALLOW_REGISTRATION="true"
+
+# -----------------------------------------------------------------------------
+# AI Providers
+# -----------------------------------------------------------------------------
+# Main provider: "openai" | "ollama" | "deepseek" | "openrouter" | "custom-openai"
+# AI_PROVIDER="openai"
+
+# Per-feature provider overrides (optional, falls back to AI_PROVIDER)
+# AI_PROVIDER_CHAT="openai"
+# AI_PROVIDER_TAGS="openai"
+# AI_PROVIDER_EMBEDDING="openai"
+
+# Model names (optional, uses provider defaults)
+# AI_MODEL_CHAT="gpt-4o-mini"
+# AI_MODEL_TAGS="gpt-4o-mini"
+# AI_MODEL_EMBEDDING="text-embedding-3-small"
+
+# OpenAI
+# OPENAI_API_KEY="sk-..."
+
+# Ollama (local)
+# OLLAMA_BASE_URL="http://localhost:11434"
+
+# Custom OpenAI-compatible endpoint
+# CUSTOM_OPENAI_API_KEY="..."
+# CUSTOM_OPENAI_BASE_URL="https://your-provider.com/v1"
+
+# -----------------------------------------------------------------------------
+# Email (at least one provider required for password reset)
+# -----------------------------------------------------------------------------
+# Resend (https://resend.com)
+# RESEND_API_KEY="re_..."
+
+# SMTP
+# SMTP_HOST="smtp.example.com"
+# SMTP_PORT="587"
+# SMTP_USER=""
+# SMTP_PASS=""
+# SMTP_FROM="noreply@example.com"
+# SMTP_SECURE="false"
+# SMTP_IGNORE_CERT="false"
+
+# -----------------------------------------------------------------------------
+# MCP (Model Context Protocol)
+# -----------------------------------------------------------------------------
+# MCP_SERVER_MODE="disabled"
+# MCP_SERVER_URL=""