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

.env.docker.example

@@ -1,80 +1,101 @@
-# ============================================
+# =============================================================================
 # Memento - Docker Environment Configuration
-# ============================================
+# =============================================================================
 # Copy this file to .env.docker and update with your values.
+# This file is read by docker-compose.yml via env_file directive.
+# cp .env.docker.example .env.docker
 
-# ============================================
-# Application URL
-# ============================================
+# =============================================================================
+# APPLICATION URL (REQUIRED)
+# =============================================================================
 # Change to your server IP or domain
+# Examples:
+#   IP:      http://192.168.1.190:3000
+#   Domain:  http://notes.yourdomain.com
+#   HTTPS:   https://notes.yourdomain.com
 NEXTAUTH_URL="http://localhost:3000"
 
-# ============================================
-# Authentication Secret
-# ============================================
+# =============================================================================
+# AUTHENTICATION SECRET (REQUIRED)
+# =============================================================================
 # Generate with: openssl rand -base64 32
 NEXTAUTH_SECRET="changethisinproduction"
 
-# ============================================
-# PostgreSQL Configuration
-# ============================================
+# =============================================================================
+# REGISTRATION
+# =============================================================================
+# Set to "false" to disable public registration (default: true)
+# ALLOW_REGISTRATION=true
+
+# =============================================================================
+# POSTGRESQL CONFIGURATION
+# =============================================================================
 POSTGRES_PORT=5432
 POSTGRES_DB=memento
 POSTGRES_USER=memento
 POSTGRES_PASSWORD=memento
 
-# ============================================
-# MCP Server Configuration
-# ============================================
+# =============================================================================
+# MCP SERVER CONFIGURATION
+# =============================================================================
 # Mode: 'stdio' (Claude Desktop, Cline) or 'sse' (N8N, HTTP)
 MCP_MODE="stdio"
 MCP_PORT="3001"
 
-# ============================================
-# AI Provider - Tags Generation
-# ============================================
+# =============================================================================
+# AI PROVIDER - TAGS GENERATION
+# =============================================================================
 # Options: ollama, openai, custom
 AI_PROVIDER_TAGS=ollama
 AI_MODEL_TAGS="granite4:latest"
 
-# ============================================
-# AI Provider - Embeddings
-# ============================================
+# =============================================================================
+# AI PROVIDER - EMBEDDINGS
+# =============================================================================
 # Options: ollama, openai, custom
 AI_PROVIDER_EMBEDDING=ollama
 AI_MODEL_EMBEDDING="embeddinggemma:latest"
 
-# ============================================
-# Ollama Configuration
-# ============================================
+# =============================================================================
+# AI PROVIDER - CHAT (optional, falls back to AI_PROVIDER_TAGS)
+# =============================================================================
+# AI_PROVIDER_CHAT=ollama
+# AI_MODEL_CHAT="granite4:latest"
+
+# =============================================================================
+# OLLAMA CONFIGURATION (if provider = ollama)
+# =============================================================================
 # Docker service:     http://ollama:11434
 # Host machine:       http://host.docker.internal:11434
 # Remote server:      http://YOUR_SERVER_IP:11434
 OLLAMA_BASE_URL="http://ollama:11434"
-OLLAMA_MODEL="granite4:latest"
 
-# ============================================
-# OpenAI Configuration
-# ============================================
+# =============================================================================
+# OPENAI CONFIGURATION (if provider = openai)
+# =============================================================================
 # OPENAI_API_KEY="sk-..."
 
-# ============================================
-# Custom OpenAI-Compatible Provider
-# ============================================
-# OpenRouter, Groq, Together AI, Mistral, etc.
+# =============================================================================
+# CUSTOM OPENAI-COMPATIBLE PROVIDER (if provider = custom)
+# =============================================================================
+# Compatible with: OpenRouter, Groq, Together AI, Mistral, etc.
+# OpenRouter:  https://openrouter.ai/api/v1
+# Groq:        https://api.groq.com/openai/v1
+# Together:    https://api.together.xyz/v1
+# Mistral:     https://api.mistral.ai/v1
 # CUSTOM_OPENAI_API_KEY="your-api-key"
 # CUSTOM_OPENAI_BASE_URL="https://openrouter.ai/api/v1"
 
-# ============================================
-# Email / SMTP Configuration
-# ============================================
+# =============================================================================
+# EMAIL / SMTP (optional, required for password reset)
+# =============================================================================
 # SMTP_HOST="smtp.gmail.com"
 # SMTP_PORT="587"
 # SMTP_USER="your-email@gmail.com"
 # SMTP_PASS="your-app-password"
 # SMTP_FROM="noreply@memento.app"
 
-# ============================================
-# Application Settings
-# ============================================
-# ALLOW_REGISTRATION=true
+# =============================================================================
+# RESEND EMAIL (alternative to SMTP, optional)
+# =============================================================================
+# RESEND_API_KEY="re_..."