Add citadel deployment

Port of ../elprice/deploy-citadel.sh: build the image locally for the
remote's architecture, ship it over SSH with docker save | docker load,
and run docker compose up -d --no-build in /home/pmp/apps/btcprice.

The deployment compose is standalone rather than joining btc-network,
because citadel runs no btcdata — BTCDATA_URL points at the instance
already running on omarchy (http://192.168.50.42:4102).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BdMTk61JgUdNH86ttMi4ce

.env.citadel

@@ -0,0 +1,8 @@
+# Environment for the citadel deployment. deploy-citadel.sh copies this to
+# $APP_DIR/.env on the target host. No secrets — btcprice has none.
+
+# btcdata runs on omarchy (192.168.50.42), publishing its Docker port 4102.
+BTCDATA_URL=http://192.168.50.42:4102
+
+# Host port; container always listens on 4040.
+BTCPRICE_PORT=4041