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>

free-port.sh

@@ -0,0 +1,25 @@
+#!/usr/bin/env bash
+# Free port 4100 by stopping Docker containers and killing local processes
+PORT=4100
+
+# Stop Docker containers bound to the port
+for cid in $(docker ps -q --filter "publish=$PORT" 2>/dev/null); do
+  name=$(docker inspect --format '{{.Name}}' "$cid" | sed 's|^/||')
+  echo "Stopping Docker container: $name ($cid)"
+  docker stop "$cid"
+done
+
+# Kill any local processes on the port
+if command -v fuser &>/dev/null; then
+  fuser -k "$PORT/tcp" 2>/dev/null && echo "Killed local process on port $PORT"
+fi
+
+# Verify
+sleep 1
+if ss -tlnp sport = :"$PORT" 2>/dev/null | grep -q "$PORT"; then
+  echo "WARNING: port $PORT still in use"
+  ss -tlnp sport = :"$PORT"
+  exit 1
+else
+  echo "Port $PORT is free"
+fi