Add shared infrastructure base stack

Postgres, Zitadel and SeaweedFS were previously duplicated or fragilely
cross-referenced across the Musify, Ping and docker-manager repos. This
consolidates them into one place: compose.yml is the production shape
(no published ports, joins infra-net), compose.dev.yml is a thin overlay
that publishes ports for local development.

.gitignore

@@ -0,0 +1,12 @@
+# Environment files (.env, .env.prod)
+.env
+.env.*
+!.env.example
+!.env.prod.example
+
+# Runtime state written by the containers themselves
+zitadel/.output/
+
+# OS
+Thumbs.db
+.DS_Store