Address PR #231 review: add docker compose check, document Caddy volumes

- Add preflight check for 'docker compose' in manage.sh (catches plugin missing)
- Document named Caddy volumes as cert storage, not user data

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

docker-compose.yml

@@ -78,6 +78,7 @@ services:
       - staging-go
 
 volumes:
+  # Named volumes for Caddy TLS certificates (not user data — managed by Caddy internally)
   caddy-data:
   caddy-data-staging:
   caddy-data-staging-go:

manage.sh

@@ -151,6 +151,13 @@ cmd_setup() {
   fi
 
   log "Docker $(docker --version | grep -oP 'version \K[^ ,]+')"
+  
+  # Check docker compose (separate check since it's a plugin/separate binary)
+  if ! docker compose version &>/dev/null; then
+    err "docker compose is required. Install Docker Desktop or docker-compose-plugin."
+    exit 1
+  fi
+  
   mark_done "docker"
 
   # ── Step 2: Config ──