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.

nip86/methods.go

@@ -1,6 +1,7 @@
 package nip86
 
 import (
+	"encoding/json"
 	"fmt"
 	"math"
 	"net"
@@ -242,6 +243,25 @@ func DecodeRequest(req Request) (MethodParams, error) {
 		}, nil
 	case "stats":
 		return Stats{}, nil
+	case "signevent":
+		if len(req.Params) == 0 {
+			return nil, fmt.Errorf("invalid number of params for '%s'", req.Method)
+		}
+
+		// params[0] is an unsigned event template {kind, content, tags,
+		// created_at}; it arrives as a decoded JSON value, so round-trip it
+		// through JSON into an Event.
+		evtJSON, err := json.Marshal(req.Params[0])
+		if err != nil {
+			return nil, fmt.Errorf("invalid event param for '%s'", req.Method)
+		}
+
+		var evt nostr.Event
+		if err := json.Unmarshal(evtJSON, &evt); err != nil {
+			return nil, fmt.Errorf("invalid event param for '%s'", req.Method)
+		}
+
+		return SignEvent{Event: evt}, nil
 	default:
 		return nil, fmt.Errorf("unknown method '%s'", req.Method)
 	}
@@ -277,6 +297,7 @@ var (
 	_ MethodParams = (*GrantAdmin)(nil)
 	_ MethodParams = (*RevokeAdmin)(nil)
 	_ MethodParams = (*Stats)(nil)
+	_ MethodParams = (*SignEvent)(nil)
 )
 
 type SupportedMethods struct{}
@@ -418,3 +439,11 @@ func (RevokeAdmin) MethodName() string { return "revokeadmin" }
 type Stats struct{}
 
 func (Stats) MethodName() string { return "stats" }
+
+// SignEvent asks the relay to sign an unsigned event template with its own
+// (self) key and return the full signed event. See NIP-86 `signevent`.
+type SignEvent struct {
+	Event nostr.Event
+}
+
+func (SignEvent) MethodName() string { return "signevent" }