Add README, update docker-compose and CLAUDE.md

Docker runs on port 4041 alongside local dev on 4000.
Include btcdata docker-compose for unified startup.

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

README.md

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+# btcprice
+
+Bitcoin price tracker UI. Serves an HTML page with live price updates streamed from [btcdata](../btcdata) via Server-Sent Events.
+
+## Quick Start
+
+```bash
+# Start btcdata first (provides price data)
+cd ../btcdata && make run
+
+# Local dev
+make run       # starts on port 4000
+make tailwind  # watch Tailwind CSS (separate terminal)
+
+# Docker (starts both btcprice + btcdata)
+docker compose up -d   # btcprice on 4041, btcdata on 4101
+```
+
+## Environment Variables
+
+| Variable | Default | Description |
+|---|---|---|
+| `PORT` | `4000` | HTTP server port |
+| `BTCDATA_URL` | `http://localhost:4100` | btcdata service URL (injected into browser JS) |
+
+## How It Works
+
+The server renders an HTML page that includes a vanilla JavaScript `EventSource` connecting directly to btcdata's SSE endpoint (`BTCDATA_URL/api/price/stream`). Price updates are rendered client-side with color-coded changes (green up, red down).
+
+## Tech Stack
+
+Clojure · Kit · Undertow · Reitit · Hiccup · Tailwind CSS v4