Add docker-compose, README, fix SSE dispatch

Use Undertow dispatch for SSE handler to avoid Ring adapter conflict.
Add docker-compose.yml (port 4101 to coexist with local dev on 4100).

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

src/clj/pmagnus/btcdata/web/routes/api.clj

@@ -28,11 +28,8 @@
     :prev_price  (when prev-price (str prev-price))
     :recorded_at (str recorded-at)}))
 
-(defn- price-stream-handler [{:keys [binance]} req]
-  (let [^HttpServerExchange exchange (:server-exchange req)
-        price-atom (:latest-price binance)
-        headers    (.getResponseHeaders exchange)
-        cors-origin (or (System/getenv "CORS_ORIGIN") "*")]
+(defn- run-sse-loop! [^HttpServerExchange exchange price-atom cors-origin]
+  (let [headers (.getResponseHeaders exchange)]
     (.setStatusCode exchange 200)
     (.put headers (HttpString. "Content-Type") "text/event-stream")
     (.put headers (HttpString. "Cache-Control") "no-cache")
@@ -59,7 +56,21 @@
         (catch Exception _)
         (finally
           (remove-watch price-atom wkey)
-          (try (.close writer) (catch Exception _)))))
+          (try (.close writer) (catch Exception _))
+          (when-not (.isComplete exchange)
+            (.endExchange exchange)))))))
+
+(defn- price-stream-handler [{:keys [binance]} req]
+  (let [^HttpServerExchange exchange (:server-exchange req)
+        price-atom  (:latest-price binance)
+        cors-origin (or (System/getenv "CORS_ORIGIN") "*")]
+    ;; Dispatch to a worker thread so the Ring adapter never processes
+    ;; the return value — Undertow handles the exchange lifecycle directly.
+    (.dispatch exchange
+      (reify io.undertow.server.HttpHandler
+        (handleRequest [_ ex]
+          (run-sse-loop! ex price-atom cors-origin))))
+    ;; Return nil; exchange is dispatched so adapter won't touch it.
     nil))
 
 (defn- latest-price-handler [{:keys [binance]} _req]