chore: Add BASE_URL environment variable documentation

- Add comment about BASE_URL in docker-compose.yml
- Document that BASE_URL should include protocol (http:// or https://)
- Provides guidance for users setting up device sync
- Kobo devices require actual network IP, not localhost

docker-compose.yml

@@ -40,6 +40,11 @@ services:
       # Application Configuration
       JWT_SECRET: ${JWT_SECRET}
       SERVER_PORT: 8765
+      # IMPORTANT: Device sync requires full URL with protocol
+      # Local: http://localhost:8765
+      # Local network: http://192.168.1.X:8765
+      # Domain: https://bookhoard.example.com
+      BASE_URL: http://localhost:${SERVER_PORT}
 
       # Rate Limiting Configuration
       TEST_MODE: ${TEST_MODE:-false}