Replace EventSource with WebSocket for price streaming

- Switch from SSE EventSource to WebSocket client connecting to
  btcdata's new /api/price/ws endpoint
- Add automatic reconnect on connection loss (3s delay)
- Update Dockerfile BTCDATA_URL to use btcdata's new Docker port 4101

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

CLAUDE.md

@@ -1,6 +1,6 @@
 # btcprice
 
-Bitcoin price tracker UI — Clojure Kit web application. Serves the HTML/Tailwind UI. Connects to btcdata for live price data via SSE.
+Bitcoin price tracker UI — Clojure Kit web application. Serves the HTML/Tailwind UI. Connects to btcdata for live price data via WebSocket.
 
 ## Build & Development Commands
 
@@ -36,9 +36,9 @@ btcprice's docker-compose includes btcdata via `include: ../btcdata/docker-compo
 - **Framework:** Kit (Integrant-based)
 - **Server:** Undertow
 - **Routing:** Reitit
-- **Templates:** Hiccup + vanilla JS EventSource
+- **Templates:** Hiccup + vanilla JS WebSocket
 - **Styling:** Tailwind CSS v4
-- **Data backend:** btcdata (separate service at BTCDATA_URL, default http://localhost:4100)
+- **Data backend:** btcdata (separate service at BTCDATA_URL, default http://localhost:4100, WebSocket at ws://localhost:4100)
 - **No database** — all data comes from btcdata
 
 ## Source Layout
@@ -58,6 +58,6 @@ src/clj/pmagnus/btcprice/
     │   └── formats.clj   # Content negotiation
     └── routes/
         ├── api.clj       # /api routes (JSON)
-        ├── ui.clj        # UI routes (HTML + EventSource)
+        ├── ui.clj        # UI routes (HTML + WebSocket)
         └── utils.clj     # Route utilities
 ```

Dockerfile

@@ -23,6 +23,6 @@ COPY --from=build /build/target/btcprice-standalone.jar /btcprice/btcprice-stand
 
 EXPOSE 4040
 ENV PORT=4040
-ENV BTCDATA_URL=http://btcdata:4100
+ENV BTCDATA_URL=http://btcdata:4101
 
 CMD ["java", "-jar", "/btcprice/btcprice-standalone.jar"]

src/clj/pmagnus/btcprice/web/routes/ui.clj

@@ -23,11 +23,15 @@
       (str
        "document.addEventListener('DOMContentLoaded', function() {\n"
        "  var btcdataUrl = '" btcdata-url "';\n"
+       "  var wsUrl = btcdataUrl.replace(/^http/, 'ws');\n"
        "  var display = document.getElementById('price-display');\n"
-       "  var source = new EventSource(btcdataUrl + '/api/price/stream');\n"
        "\n"
-       "  source.addEventListener('price-update', function(e) {\n"
+       "  function connect() {\n"
+       "    var ws = new WebSocket(wsUrl + '/api/price/ws');\n"
+       "\n"
+       "    ws.onmessage = function(e) {\n"
        "      var d = JSON.parse(e.data);\n"
+       "      if (d.ping) return;\n"
        "      var price = parseFloat(d.price);\n"
        "      var prevPrice = d.prev_price ? parseFloat(d.prev_price) : null;\n"
        "      var color = 'text-gray-900';\n"
@@ -44,12 +48,18 @@
        "        '<p class=\"text-4xl font-bold ' + color + '\">' + fmt + '</p>' +\n"
        "        '<p class=\"text-xs text-gray-400 mt-2\">Updated ' + timeStr + '</p>' +\n"
        "        '</div></div>';\n"
-       "  });\n"
+       "    };\n"
        "\n"
-       "  source.onerror = function() {\n"
+       "    ws.onclose = function() {\n"
        "      display.innerHTML = '<div class=\"bg-white rounded-2xl shadow-sm border border-gray-200 p-6\">' +\n"
        "        '<p class=\"text-center text-red-400 text-sm\">Connection lost. Reconnecting...</p></div>';\n"
+       "      setTimeout(connect, 3000);\n"
        "    };\n"
+       "\n"
+       "    ws.onerror = function() {};\n"
+       "  }\n"
+       "\n"
+       "  connect();\n"
        "});\n"))]))
 
 (defn- ui-routes [opts]