Add signevent method to NIP-86 (nip86 + khatru)

Implements the signevent capability from nostr-protocol/nips#2389: relay admins
can ask the relay to sign an unsigned event template with its own (self) key.

- nip86: SignEvent MethodParams type + "signevent" decode case (round-trips the
  {kind, content, tags, created_at} param into a nostr.Event).
- khatru: RelayManagementAPI.SignEvent hook + dispatch returning the signed event
  as the result.

khatru/nip86.go

@@ -43,6 +43,7 @@ type RelayManagementAPI struct {
 	Stats                       func(ctx context.Context) (nip86.Response, error)
 	GrantAdmin                  func(ctx context.Context, pubkey nostr.PubKey, methods []string) error
 	RevokeAdmin                 func(ctx context.Context, pubkey nostr.PubKey, methods []string) error
+	SignEvent                   func(ctx context.Context, event nostr.Event) (nostr.Event, error)
 	Generic                     func(ctx context.Context, request nip86.Request) (nip86.Response, error)
 }
 
@@ -346,6 +347,14 @@ func (rl *Relay) HandleNIP86(w http.ResponseWriter, r *http.Request) {
 			} else {
 				resp.Result = result
 			}
+		case nip86.SignEvent:
+			if rl.ManagementAPI.SignEvent == nil {
+				resp.Error = fmt.Sprintf("method %s not supported", thing.MethodName())
+			} else if result, err := rl.ManagementAPI.SignEvent(ctx, thing.Event); err != nil {
+				resp.Error = err.Error()
+			} else {
+				resp.Result = result
+			}
 		default:
 			if rl.ManagementAPI.Generic == nil {
 				resp.Error = fmt.Sprintf("method '%s' not known", mp.MethodName())