deploy: split into dev/prod with separate Shopify configs and containers

- shopify.app.toml -> shopify.app.dev.toml (domain: invoice-app-dev.linumiq.com)
- New shopify.app.prod.toml will be created via shopify app config link --config prod
- docker-compose split into deploy/docker-compose.{dev,prod}.yml with distinct
  container names (linumiq-invoice-{dev,prod}), images, env files and bind mounts
- Caddyfile snippet maps both subdomains to their respective containers
- .env.{dev,prod}.example templates committed in deploy/
- deploy/README.md documents the layout and day-to-day workflow

deploy/Caddyfile.snippet

@@ -1,10 +1,18 @@
 # Append to your existing Caddyfile (or include via `import`).
-# DNS A/AAAA record for invoice-app.linumiq.com must point to this server first,
-# otherwise Caddy will fail to obtain a Let's Encrypt certificate.
+# DNS A/AAAA records for both subdomains must point to this server first
+# (a wildcard *.linumiq.com record is sufficient).
+#
+# Caddy runs in Docker on the `caddy_net` network and reaches each app by
+# container name (the apps do not publish host ports).
 
-# Caddy runs in Docker on the `caddy_net` network and reaches the app by
-# container name (the app does not publish a host port).
+# DEV — installed on linumiq-dev.myshopify.com
+invoice-app-dev.linumiq.com {
+	encode zstd gzip
+	reverse_proxy linumiq-invoice-dev:3000
+}
+
+# PROD — installed on shop.linumiq.com (5aiizq-ti.myshopify.com)
 invoice-app.linumiq.com {
 	encode zstd gzip
-	reverse_proxy linumiq-invoice:3000
+	reverse_proxy linumiq-invoice-prod:3000
 }