Discover applications on the cluster and the host for backups

Kubernetes workloads are grouped by their Helm instance label into
applications, each offering what is worth backing up: data volumes, a
PostgreSQL dump instead of the database's own volume, and optionally the
namespace manifests. Caches are listed but not preselected. Host
applications come from running systemd services that declare a state or
working directory. Selecting components creates one strategy each.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/crates/domain/src/lib.rs

@@ -1,5 +1,6 @@
 //! Domain layer: entities, value objects, errors and the ports (traits) the application
 //! layer depends on. No I/O here.
+pub mod application;
 pub mod auth;
 pub mod backup;
 pub mod cluster;