Replace SSE price stream with WebSocket, use separate Docker port

- Replace SSE endpoint (/api/price/stream) with WebSocket (/api/price/ws)
  using ring-undertow-adapter's built-in WebSocket support
- Change Docker internal port from 4100 to 4101 to avoid conflicts with
  local dev (make run still uses 4100)
- Add free-port.sh utility script

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -3,9 +3,9 @@ services:
     build: .
     restart: unless-stopped
     ports:
-      - "4101:4100"
+      - "4101:4101"
     environment:
-      BTCDATA_PORT: "4100"
+      BTCDATA_PORT: "4101"
       JDBC_URL: "jdbc:postgresql://postgres:5432/btcprod?user=postgres&password=ratata,123"
       CORS_ORIGIN: "http://localhost:4041"
     extra_hosts: