feat(smtp): complete native Lua SMTP integration for production mail delivery

- Add native Lua SMTP client with SSL/TLS support for mail.dragons-at-work.de:465
- Implement POST /v1/mail/send endpoint with real email delivery functionality
- Add environment variable integration (SMTP_*) for secure credential management
- Add comprehensive input validation and error handling for mail requests
- Add health check endpoint with SMTP configuration status reporting
- Add multi-line SMTP response handling for robust server communication
- Add request ID tracking system for debugging and monitoring
- Update start.sh script for automatic .env loading and dependency checking
- Add complete testing suite for SMTP functionality verification

This completes the Week 2 Challenge migration from Go to pure Lua HTTP server
with full production-ready SMTP capabilities. The implementation eliminates all
Google/corporate dependencies while achieving superior performance (18ms response
time) and maintaining digital sovereignty principles.

Real mail delivery confirmed: test email successfully sent to admin@dragons-at-work.de
Ready for Hugo website integration and production deployment with security layer.

Closes #65

furt-lua/scripts/setup_env.sh

@@ -0,0 +1,101 @@
+#!/bin/bash
+# furt-lua/scripts/setup_env.sh
+# Add SMTP environment variables to existing .env (non-destructive)
+
+set -e
+
+# Colors for output
+GREEN='\033[0;32m'
+YELLOW='\033[1;33m'
+RED='\033[0;31m'
+NC='\033[0m' # No Color
+
+echo -e "${GREEN}=== Furt SMTP Environment Setup ===${NC}"
+
+# Navigate to furt project root
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+PROJECT_ROOT="$(dirname "$(dirname "$SCRIPT_DIR")")"
+ENV_FILE="$PROJECT_ROOT/.env"
+
+echo -e "${YELLOW}Project root:${NC} $PROJECT_ROOT"
+echo -e "${YELLOW}Environment file:${NC} $ENV_FILE"
+
+# Check if .env exists
+if [ ! -f "$ENV_FILE" ]; then
+    echo -e "${YELLOW}Creating new .env file...${NC}"
+    cat > "$ENV_FILE" << 'EOF'
+# Dragons@Work Project Environment Variables
+
+# Furt SMTP Configuration for mail.dragons-at-work.de
+SMTP_HOST="mail.dragons-at-work.de"
+SMTP_PORT="465"
+SMTP_USERNAME="your_email@dragons-at-work.de"
+SMTP_PASSWORD="your_smtp_password"
+SMTP_FROM="noreply@dragons-at-work.de"
+SMTP_TO="michael@dragons-at-work.de"
+EOF
+    echo -e "${GREEN}[OK] Created new .env file${NC}"
+    echo -e "${YELLOW}[EDIT] Please edit:${NC} nano $ENV_FILE"
+    exit 0
+fi
+
+echo -e "${GREEN}[OK] Found existing .env file${NC}"
+
+# Check if SMTP variables already exist
+smtp_username_exists=$(grep -c "^SMTP_USERNAME=" "$ENV_FILE" 2>/dev/null || echo "0")
+smtp_password_exists=$(grep -c "^SMTP_PASSWORD=" "$ENV_FILE" 2>/dev/null || echo "0")
+
+if [ "$smtp_username_exists" -gt 0 ] && [ "$smtp_password_exists" -gt 0 ]; then
+    echo -e "${GREEN}[OK] SMTP variables already configured${NC}"
+    
+    # Load and show current values
+    source "$ENV_FILE"
+    echo -e "${YELLOW}Current SMTP User:${NC} ${SMTP_USERNAME:-NOT_SET}"
+    echo -e "${YELLOW}Current SMTP Password:${NC} ${SMTP_PASSWORD:+[CONFIGURED]}${SMTP_PASSWORD:-NOT_SET}"
+    
+    echo ""
+    echo -e "${YELLOW}To update SMTP settings:${NC} nano $ENV_FILE"
+    exit 0
+fi
+
+# Add missing SMTP variables
+echo -e "${YELLOW}Adding SMTP configuration to existing .env...${NC}"
+
+# Add section header if not present
+if ! grep -q "SMTP_" "$ENV_FILE" 2>/dev/null; then
+    echo "" >> "$ENV_FILE"
+    echo "# Furt SMTP Configuration for mail.dragons-at-work.de" >> "$ENV_FILE"
+fi
+
+# Add username if missing
+if [ "$smtp_username_exists" -eq 0 ]; then
+    echo "SMTP_HOST=\"mail.dragons-at-work.de\"" >> "$ENV_FILE"
+    echo "SMTP_PORT=\"465\"" >> "$ENV_FILE"
+    echo "SMTP_USERNAME=\"your_email@dragons-at-work.de\"" >> "$ENV_FILE"
+    echo -e "${GREEN}[OK] Added SMTP_HOST, SMTP_PORT, SMTP_USERNAME${NC}"
+fi
+
+# Add password if missing
+if [ "$smtp_password_exists" -eq 0 ]; then
+    echo "SMTP_PASSWORD=\"your_smtp_password\"" >> "$ENV_FILE"
+    echo "SMTP_FROM=\"noreply@dragons-at-work.de\"" >> "$ENV_FILE"
+    echo "SMTP_TO=\"michael@dragons-at-work.de\"" >> "$ENV_FILE"
+    echo -e "${GREEN}[OK] Added SMTP_PASSWORD, SMTP_FROM, SMTP_TO${NC}"
+fi
+
+echo -e "${GREEN}[OK] SMTP configuration added to .env${NC}"
+echo ""
+echo -e "${YELLOW}Next steps:${NC}"
+echo "1. Edit SMTP credentials: nano $ENV_FILE"
+echo "2. Set your actual email@dragons-at-work.de in SMTP_USERNAME"
+echo "3. Set your actual SMTP password in SMTP_PASSWORD"
+echo "4. Test with: ./scripts/start.sh"
+
+echo ""
+echo -e "${YELLOW}Current .env content:${NC}"
+echo "==================="
+cat "$ENV_FILE"
+echo "==================="
+echo ""
+echo -e "${GREEN}Ready for SMTP testing!${NC}"
+