feat(monitoring/site-mappings): globalize site_domain_mappings

Drop tenant scoping on site_domain_mappings now that the citation query
resolves mappings without tenant filtering. Add a unique index on
registrable_domain plus an updated_at column, and switch the dev-seed
upsert to use ON CONFLICT (registrable_domain). Adds a SiteDomainMapping
type to ops/domain for the upcoming console management UI.

server/internal/ops/domain/types.go

@@ -63,3 +63,13 @@ type AuditLogFilter struct {
 	Limit      int
 	Offset     int
 }
+
+type SiteDomainMapping struct {
+	ID                 int64
+	RegistrableDomain string
+	SiteKey            string
+	SiteName           string
+	IsActive           bool
+	CreatedAt          time.Time
+	UpdatedAt          time.Time
+}