Replace 15+ concrete type assertions with capability interfaces in cmd/server/main.go #59

Problem

cmd/server/main.go iterates the service registry using 15+ type assertions like if gen, ok := svc.(*module.OpenAPIGenerator); ok { ... }. This forces the server to know about every specific module type at compile time, breaking plugin isolation.

Fix Direction

Define capability interfaces (e.g. SchemaRegistrar, WorkflowStoreProvider) that modules opt into. Iterate the service registry checking for interface satisfaction instead of type identity:

for _, svc := range app.SvcRegistry() {
    if registrar, ok := svc.(SchemaRegistrar); ok {
        registrar.RegisterSchemas()
    }
}

Note

This item depends on the serverApp god object refactor (see related issue) being merged first, as both touch cmd/server/main.go.

Context

See docs/DEFERRED_ISSUES.md — Architecture section.