Move shared infrastructure to a separate Infrastructure repo

Postgres, Zitadel, SeaweedFS, RabbitMQ and Jaeger no longer run as part
of this stack. They're shared services (used by Musify, Ping and
docker-manager) that now live in a separate Infrastructure repository
and must already be running.

deploy/compose.yml keeps only what's specific to Musify: its own
database connection, its own OIDC app and storage bucket (created
against the shared Zitadel/SeaweedFS via zitadel-init/seaweedfs-init),
and the application containers. A new wait-for-postgres one-shot
replaces the in-file Postgres health check, which no longer works now
that Postgres lives in a different compose project.

compose.prod.yml joins Infrastructure's infra-net network instead of
running its own copies. Docs and READMEs updated to match.

README.md

@@ -29,9 +29,11 @@ dedicated streaming gateway.
 **Web client**: SvelteKit 2 on Svelte 5 (runes), TypeScript, Tailwind 4.
 The session lives on the server, so the backend access token never reaches the browser.
 
-**Infrastructure**: the whole stack runs from Docker Compose (dev and
-production overlays), with Postgres, RabbitMQ, SeaweedFS, Zitadel and Jaeger
-alongside the apps.
+**Infrastructure**: Musify's own apps run from Docker Compose (dev and
+production overlays). Postgres, RabbitMQ, SeaweedFS, Zitadel and Jaeger are
+shared infrastructure that lives in a separate `Infrastructure` git
+repository, used by Musify and by other projects that need the same OIDC
+provider and object storage.
 
 ## Project layout
 
@@ -42,6 +44,9 @@ deploy/       Docker Compose stacks for dev and production
 docs/         architecture notes, one file per area
 ```
 
+Postgres, Zitadel, SeaweedFS, RabbitMQ and Jaeger are not part of this
+layout. They live in the separate `Infrastructure` repository instead.
+
 ## Digging deeper
 
 [docs/](docs/README.md) covers how each piece works and why: architecture,