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.

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app/backend/migrations_goose/20261114000000_node_default_domain.sql

@@ -0,0 +1,8 @@
+-- +goose Up
+-- Per-node base domain: services pinned/spread onto an agent with
+-- default_domain get auto-generated hostnames (<service>.<domain>) when
+-- they have no explicit service_domains rows.
+ALTER TABLE node_agents ADD COLUMN IF NOT EXISTS default_domain VARCHAR(255) NOT NULL DEFAULT '';
+
+-- +goose Down
+ALTER TABLE node_agents DROP COLUMN IF EXISTS default_domain;