Initial commit: btcdata — BTC data backend service

Split from btcprice monolith. Owns DB, migrations, Binance WebSocket,
Kraken OHLC pollers, and exposes a JSON API with SSE price stream.

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

.gitignore

@@ -0,0 +1,17 @@
+/target
+/classes
+/checkouts
+profiles.clj
+pom.xml
+pom.xml.asc
+*.jar
+*.class
+/.lein-*
+/.nrepl-port
+/.cpcache
+/.clj-kondo
+/log
+.DS_Store
+*.log
+/.env
+/token