feat(federation): hook outbox and mutation log into DM message and reaction handlers

Wire appendMutationLog + queueOutboxEvent + buildRelayPayload into all
DM mutation paths so federation peers receive relay events:

- REST: POST /api/dm/:id/messages, PATCH /api/dm/messages/:id,
  DELETE /api/dm/messages/:id
- WebSocket: dm_message_create, dm_message_edit, dm_message_delete,
  reaction_add (DM path), reaction_remove (DM path)
- Fix buildRelayPayload parameter types to accept optional replyToId
  and editedAt (matching DmMessageWithUser's optional fields)

packages/server/src/utils/federationOutbox.ts

@@ -217,8 +217,8 @@ export function buildRelayPayload(
   message: {
     id: string;
     content: string | null;
-    replyToId: string | null;
-    editedAt: number | null;
+    replyToId?: string | null;
+    editedAt?: number | null;
     createdAt: number;
   },
   user: {
@@ -232,8 +232,8 @@ export function buildRelayPayload(
     homeUserId: user.homeUserId || user.id,
     homeInstance: user.homeInstance || '',
     content: message.content,
-    replyToId: message.replyToId,
-    editedAt: message.editedAt,
+    replyToId: message.replyToId ?? null,
+    editedAt: message.editedAt ?? null,
     createdAt: message.createdAt,
   };
 }