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>

CLAUDE.md

@@ -1,6 +1,6 @@
 # btcdata
 
-Bitcoin data backend — Clojure Kit application. Fetches BTC prices from Binance WebSocket and Kraken HTTP, stores in PostgreSQL, exposes a JSON API with SSE price stream.
+Bitcoin data backend — Clojure Kit application. Fetches BTC prices from Binance WebSocket and Kraken HTTP, stores in PostgreSQL, exposes a JSON API with WebSocket price stream.
 
 ## Build & Development Commands
 
@@ -44,7 +44,7 @@ Docker runs alongside local dev on different ports and databases:
 ## API Endpoints
 
 - `GET /api/health` — Health check
-- `GET /api/price/stream` — SSE stream with JSON price updates
+- `GET /api/price/ws` — WebSocket endpoint for live JSON price updates
 - `GET /api/price/latest` — Latest price as JSON
 
 ## Source Layout
@@ -67,6 +67,6 @@ src/clj/pmagnus/btcdata/
     │   ├── exception.clj # Exception handling
     │   └── formats.clj   # Content negotiation
     └── routes/
-        ├── api.clj       # /api routes (JSON + SSE)
+        ├── api.clj       # /api routes (JSON + WebSocket)
         └── utils.clj     # Route utilities
 ```