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.

postgres-init/010-create-app-databases.sql

@@ -0,0 +1,9 @@
+-- Runs once, only on first init of the postgres_data volume.
+-- Zitadel provisions its own "zitadel" database itself, from the
+-- ZITADEL_DATABASE_POSTGRES_ADMIN_* credentials in compose.yml — it does not
+-- need to be created here. This script only creates the plain application
+-- databases each project's own connection string points at.
+
+CREATE DATABASE musify_db;
+CREATE DATABASE ping_db;
+CREATE DATABASE dockermanager_db;