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/domains_test.go

@@ -0,0 +1,26 @@
+package api
+
+import "testing"
+
+func TestDNSLabel(t *testing.T) {
+	cases := map[string]string{
+		"my-app":        "my-app",
+		"My App_2":      "my-app-2",
+		"  web  ":       "web",
+		"__weird__name": "weird-name",
+		"---":           "",
+		"":              "",
+	}
+	for in, want := range cases {
+		if got := dnsLabel(in); got != want {
+			t.Errorf("dnsLabel(%q) = %q, want %q", in, got, want)
+		}
+	}
+}
+
+func TestDNSLabelLengthCap(t *testing.T) {
+	long := "a" + string(make([]byte, 0)) + "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef"
+	if got := dnsLabel(long); len(got) > 63 {
+		t.Fatalf("label over 63 chars: %d", len(got))
+	}
+}