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/api/types.go

@@ -45,6 +45,9 @@ type Zone struct {
 	// (POST /zones/{zone}/fill-ptr adds them).
 	Prefix     string `json:"prefix,omitempty"`
 	PTRMissing int    `json:"ptr_missing,omitempty"`
+	// PTRMissingList are the first 25 of them: the PTR records adding
+	// them would create.
+	PTRMissingList []PTRFill `json:"ptr_missing_list,omitempty"`
 	// Pending is set when listing with a changeset: add, change or delete.
 	Pending string `json:"pending,omitempty"`
 }
@@ -76,6 +79,17 @@ type View struct {
 	Limited bool `json:"limited,omitempty"`
 }
 
+// PTRFill is a PTR record a reverse zone lacks: Name (the PTR owner) for
+// Address would give Target. Others are its other names; From the view
+// whose record names it, when the zone's own view has none.
+type PTRFill struct {
+	Address string   `json:"address"`
+	Name    string   `json:"name"`
+	Target  string   `json:"target"`
+	Others  []string `json:"others,omitempty"`
+	From    string   `json:"from,omitempty"`
+}
+
 // ViewColor sets the colour the web UI shows a view in.
 type ViewColor struct {
 	Color string `json:"color"`