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.

docs/projects.md

@@ -69,6 +69,8 @@ real Postgres + SeaweedFS), and `Musify.StreamingGateway.Tests`. Details in
 
 ## Other folders
 
-- **`deploy/`**: self-contained Docker Compose stacks (dev and prod). See
+- **`deploy/`**: Musify's own Docker Compose stacks (dev and prod). The
+  shared infrastructure (Postgres, Zitadel, SeaweedFS, RabbitMQ, Jaeger)
+  lives in the separate `Infrastructure` repo instead. See
   [../deploy/README.md](../deploy/README.md).
 - **`docs/`**: this documentation.