feat(nodes): per-node default domain with auto service hostnames (#65)

node_agents.default_domain gives each agent a base domain. Services
placed on it (explicit pin or spread) with no explicit service_domains
get <name>.<default_domain> — resolved in serviceDomainNames so edge
routing, wake placeholders, and public_url all see the same names.
Spread services union every eligible node's base domain; explicit
domains always win.

Surfaces: PUT /agents/:id {default_domain} (hostname-validated), CLI
nodes update --domain, MCP arg docs, OpenAPI + regenerated types,
node-detail inline editor.

Also marks the already-shipped backup-import roadmap item.

Generated with [Devin](https://devin.ai)

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/backend/internal/api/runtime.go

@@ -659,6 +659,8 @@ func liveServiceStatus(c *gin.Context, db *database.DB, service *Service) {
 	persistPublishedPort(db, service.ID, state.Ports)
 	if service.Domain != "" {
 		service.PublicURL = "https://" + service.Domain
+	} else if auto := serviceAutoDomains(db, service.ID); len(auto) > 0 {
+		service.PublicURL = "https://" + auto[0]
 	} else if len(state.URLs) > 0 {
 		service.PublicURL = rewriteLoopbackURL(state.URLs[0], requestHostname(c))
 	}