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
 ```