Certificate export, missing PTR list, simpler "Used for"

- Certificates can be exported for use outside Jarl: Export… on the
  Certificates page (download or copy), jarl cert export, and
  GET /certificates/{name}/pem?part=fullchain|cert|key|bundle.
  Administrators only; key exports are audited.
- A reverse zone's missing-PTR banner lists the addresses and names
  (ptr_missing_list in the zone API).
- "Used for" shows DoT/DoH for named, and "not used by Jarl" instead of
  "nothing yet".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/server/dsat_test.go

@@ -80,3 +80,16 @@ func TestOneViewToken(t *testing.T) {
 	// The admin still has to say which.
 	e.do("GET", "/zones/adbraun.lol", e.token, nil, 400)
 }
+
+// A reverse zone lists the PTR records it lacks, not just how many.
+func TestPTRMissingList(t *testing.T) {
+	e := newAPIEnv(t)
+	e.do("POST", "/zones", e.token, map[string]any{"name": "adbraun.lol", "nameservers": []string{"ns1.example.net."},
+		"records": []map[string]any{{"name": "nas", "type": "A", "data": "10.60.60.5"}}}, 201)
+	e.do("POST", "/zones", e.token, map[string]any{"name": "10.60.60.0/24", "nameservers": []string{"ns1.example.net."}}, 201)
+	z := e.do("GET", "/zones/60.60.10.in-addr.arpa", e.token, nil, 200)
+	list, _ := z["ptr_missing_list"].([]any)
+	if z["ptr_missing"] != float64(1) || len(list) != 1 || list[0].(map[string]any)["address"] != "10.60.60.5" || list[0].(map[string]any)["target"] != "nas.adbraun.lol." {
+		t.Errorf("ptr_missing %v, list %v", z["ptr_missing"], z["ptr_missing_list"])
+	}
+}