fix: make service config optional in API design (#325)

Remove config from Required fields in the Goa DSL and add omitempty
to the JSON tag. The custom validation layer already handles nil
configs with sensible defaults, but the generated validation was
rejecting requests before custom parsing could run.

Normalize nil config to an empty map in the API conversion layer so
downstream code never needs nil checks.

Author: rshoemaker

api/apiv1/design/database.go

@@ -199,7 +199,7 @@ var ServiceSpec = g.Type("ServiceSpec", func() {
 			"llm_model":      "gpt-4",
 			"openai_api_key": "sk-...",
 		})
-		g.Meta("struct:tag:json", "config")
+		g.Meta("struct:tag:json", "config,omitempty")
 	})
 	g.Attribute("cpus", g.String, func() {
 		g.Description("The number of CPUs to allocate for this service. It can include the SI suffix 'm', e.g. '500m' for 500 millicpus. Defaults to container defaults if unspecified.")
@@ -225,7 +225,7 @@ var ServiceSpec = g.Type("ServiceSpec", func() {
 		g.Meta("struct:tag:json", "database_connection,omitempty")
 	})
 
-	g.Required("service_id", "service_type", "version", "host_ids", "config")
+	g.Required("service_id", "service_type", "version", "host_ids")
 })
 
 var BackupRepositorySpec = g.Type("BackupRepositorySpec", func() {