Update doc

.env.example

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+# Copy to .env and fill in. .env is gitignored.
+# The Makefile does `include .env`, so this file must exist before `make run`.
+
+# --- Local dev (make run) ---
+# btcdata instance the server proxies WebSockets to. The code falls back to
+# http://localhost:4100 when unset, but the Makefile needs the file to exist.
+BTCDATA_URL=http://localhost:4100
+
+# Used by `make run-local` to point at a btcdata on this machine while
+# BTCDATA_URL above points elsewhere (e.g. a deployed instance).
+BTCDATA_LOCAL_URL=http://localhost:4100
+
+# --- Docker (docker compose up -d) ---
+# Port btcdata listens on inside btc-network. Must match DOCKER_BTCDATA_PORT
+# in ../btcdata/.env.
+DOCKER_BTCDATA_PORT=4102