Add .env.example for relay server configuration

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

macropad-relay/.env.example

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+# MacroPad Relay Server Configuration
+# Copy this file to .env and adjust values as needed
+
+# Server
+PORT=3000
+HOST=0.0.0.0
+NODE_ENV=production
+
+# Security
+BCRYPT_ROUNDS=10
+
+# Session ID length (default: 6 characters)
+SESSION_ID_LENGTH=6
+
+# Rate limiting
+RATE_LIMIT_WINDOW_MS=900000
+RATE_LIMIT_MAX=100
+
+# WebSocket timeouts (in milliseconds)
+PING_INTERVAL=30000
+REQUEST_TIMEOUT=30000
+
+# Logging (error, warn, info, debug)
+LOG_LEVEL=info
+
+# Data directory (default: ./data)
+# DATA_DIR=/path/to/data