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>

docker-compose.override.yml

@@ -1,6 +1,7 @@
-# Local development only. Compose merges this automatically; it is not used on
-# the Pi, where Caddy reaches the container over the shared Docker network and
-# no host port is published.
+# Local development only. Compose merges this automatically whenever
+# COMPOSE_FILE is unset — which is why the Pi's .env sets COMPOSE_FILE to
+# deploy/docker-compose.pi.yml instead (docs/PI.md): there, Caddy reaches the
+# container over the shared Docker network and no host port is published.
 services:
   postgres:
     ports: