feat: implement GET /api/v1/domains and /api/v1/hostnames endpoints

Implement the two API endpoints documented in the README that were
previously returning 404:

- GET /api/v1/domains: Returns configured domains with their
  discovered nameservers, last check time, and status (ok/pending).

- GET /api/v1/hostnames: Returns configured hostnames with
  per-nameserver DNS records, status, and last check time.

Both endpoints read from the existing state store and config,
requiring no new dependencies. Nameserver results in the hostnames
endpoint are sorted for deterministic output.

Closes #67

internal/handlers/domains.go

@@ -0,0 +1,60 @@
+package handlers
+
+import (
+	"net/http"
+	"time"
+)
+
+// domainResponse represents a single domain in the API response.
+type domainResponse struct {
+	Domain      string   `json:"domain"`
+	Nameservers []string `json:"nameservers,omitempty"`
+	LastChecked string   `json:"lastChecked,omitempty"`
+	Status      string   `json:"status"`
+}
+
+// domainsResponse is the top-level response for GET /api/v1/domains.
+type domainsResponse struct {
+	Domains []domainResponse `json:"domains"`
+}
+
+// HandleDomains returns the configured domains and their status.
+func (h *Handlers) HandleDomains() http.HandlerFunc {
+	return func(
+		writer http.ResponseWriter,
+		request *http.Request,
+	) {
+		configured := h.config.Domains
+		snapshot := h.state.GetSnapshot()
+
+		domains := make(
+			[]domainResponse, 0, len(configured),
+		)
+
+		for _, domain := range configured {
+			dr := domainResponse{
+				Domain: domain,
+				Status: "pending",
+			}
+
+			ds, ok := snapshot.Domains[domain]
+			if ok {
+				dr.Nameservers = ds.Nameservers
+				dr.Status = "ok"
+
+				if !ds.LastChecked.IsZero() {
+					dr.LastChecked = ds.LastChecked.
+						Format(time.RFC3339)
+				}
+			}
+
+			domains = append(domains, dr)
+		}
+
+		h.respondJSON(
+			writer, request,
+			&domainsResponse{Domains: domains},
+			http.StatusOK,
+		)
+	}
+}