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.

.env.example

@@ -0,0 +1,36 @@
+# Development configuration for compose.yml + compose.dev.yml.
+# Copy to .env and adjust.
+#
+#   cp .env.example .env
+
+# --- PostgreSQL (shared instance, one database per project) -----------------
+POSTGRES_USER=postgres
+POSTGRES_PASSWORD=postgres
+POSTGRES_HOST_PORT=5432
+
+# --- Zitadel (OIDC) ----------------------------------------------------------
+# The masterkey MUST be exactly 32 characters. host.docker.internal resolves
+# to the same address from the browser, from apps on the host and from
+# inside every project's containers, which is what makes the OIDC issuer
+# valid everywhere without a shared Docker network in dev.
+ZITADEL_MASTERKEY=MasterkeyNeedsToHave32Characters
+ZITADEL_EXTERNAL_DOMAIN=host.docker.internal
+ZITADEL_EXTERNAL_PORT=8080
+ZITADEL_EXTERNAL_SECURE=false
+ZITADEL_DATABASE_PASSWORD=zitadel
+# Console login: <username>@zitadel.<external domain>
+ZITADEL_ADMIN_USERNAME=admin
+ZITADEL_ADMIN_PASSWORD=Password1!
+
+# --- SeaweedFS (S3) ----------------------------------------------------------
+S3_ACCESS_KEY=admin_access_key
+S3_SECRET_KEY=admin_secret_key
+
+# --- RabbitMQ -----------------------------------------------------------------
+RABBITMQ_DEFAULT_USER=guest
+RABBITMQ_DEFAULT_PASS=guest
+
+# --- pgAdmin (optional, `--profile tools`) -----------------------------------
+PGADMIN_DEFAULT_EMAIL=admin@infra.dev
+PGADMIN_DEFAULT_PASSWORD=admin
+PGADMIN_HOST_PORT=5050