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

docker-compose.citadel.yml

@@ -0,0 +1,11 @@
+# Deployment compose for citadel. Copied to $APP_DIR/docker-compose.yml by
+# deploy-citadel.sh, which also copies .env.citadel to $APP_DIR/.env.
+# Standalone: btcdata is remote, so no btc-network here.
+services:
+  btcprice:
+    image: btcprice:latest
+    restart: unless-stopped
+    ports:
+      - "${BTCPRICE_PORT:-4041}:4040"
+    environment:
+      BTCDATA_URL: "${BTCDATA_URL:?set BTCDATA_URL in .env}"