SYS-18: add production deployment stack for karbe.cosmolan.fr (Stripe test)

- enable Next.js standalone output and add Docker/Caddy production stack
- add production env template and deployment runbook
- add healthcheck endpoint for container supervision
- fix existing lint/type blockers discovered during validation

Co-Authored-By: Paperclip <noreply@paperclip.ing>

docker/Caddyfile

@@ -0,0 +1,14 @@
+{
+  email ops@cosmolan.fr
+}
+
+karbe.cosmolan.fr {
+  encode zstd gzip
+  reverse_proxy app:3000
+
+  header {
+    Strict-Transport-Security "max-age=31536000; includeSubDomains; preload"
+    X-Content-Type-Options "nosniff"
+    Referrer-Policy "strict-origin-when-cross-origin"
+  }
+}