feat: Add presence heartbeat for Matrix online status

- matrix-gateway: POST /internal/matrix/presence/online endpoint
- usePresenceHeartbeat hook with activity tracking
- Auto away after 5 min inactivity
- Offline on page close/visibility change
- Integrated in MatrixChatRoom component

backend/infra/config/env.ts

@@ -0,0 +1,21 @@
+/**
+ * Environment Configuration
+ * Load and validate environment variables
+ */
+
+export const config = {
+  port: parseInt(process.env.PORT || '3000', 10),
+  nodeEnv: process.env.NODE_ENV || 'development',
+  apiBaseUrl: process.env.API_BASE_URL || 'http://localhost:3000',
+  
+  // Database
+  dbUrl: process.env.DATABASE_URL || 'postgresql://localhost:5432/microdao',
+  
+  // Auth
+  jwtSecret: process.env.JWT_SECRET || 'change-me-in-production',
+  
+  // Wallet/Chain (future)
+  chainRpcUrl: process.env.CHAIN_RPC_URL || '',
+};
+
+