Add a setup guide for the Pi, and a compose file its .env selects

docs/PI.md goes from a Pi with Docker to a working claude.ai connector:
configuration, the first token, Caddy, DNS, the VPS forwarding raw TCP,
checks from outside, connecting the clients, the monthly token, updates,
backups and troubleshooting.

docker-compose.override.yml is tracked, so Compose would have merged it on
the Pi as well — publishing ports and switching the crawl timer off. The
Pi's .env sets COMPOSE_FILE to add deploy/docker-compose.pi.yml instead,
which joins the existing Caddy network by name and builds DATABASE_URL, so
nothing tracked needs editing there. Postgres moves to a private network
shared only with the server.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

deploy/docker-compose.pi.yml

@@ -0,0 +1,23 @@
+# The Pi's additions to docker-compose.yml — see docs/PI.md.
+#
+# Selected by one line in the Pi's .env, so plain `docker compose` commands use it:
+#
+#   COMPOSE_FILE=docker-compose.yml:deploy/docker-compose.pi.yml
+#
+# Setting COMPOSE_FILE also keeps docker-compose.override.yml out. That file is
+# for local development only, and Compose would otherwise merge it silently: it
+# publishes ports and turns the crawl timer off.
+
+services:
+  schulcloud-mcp:
+    environment:
+      # The bundled Postgres, by name on the private backend network. Overrides
+      # any DATABASE_URL in .env.
+      DATABASE_URL: postgresql://schulcloud:${POSTGRES_PASSWORD:?set POSTGRES_PASSWORD in .env}@postgres:5432/schulcloud
+
+networks:
+  caddy:
+    # Join the network the Pi's Caddy container is already on, so Caddy reaches
+    # this server by name and no host port is published.
+    external: true
+    name: ${CADDY_NETWORK:?set CADDY_NETWORK in .env to the Docker network of your Caddy container}