Add structured JSON evidence export workflow

src/background.js

@@ -37,6 +37,10 @@ async function handleVendorGetMessage(message, sender) {
     return handleFetchOfficialGvlMessage();
   }
 
+  if (message.type === "export_evidence_json") {
+    return handleExportEvidenceJsonMessage();
+  }
+
   if (message.type === "start_evidence_maintenance_session") {
     return startEvidenceMaintenanceSession(message?.payload?.source);
   }
@@ -169,6 +173,13 @@ async function handleGetEvidenceRetentionStatusMessage() {
   };
 }
 
+async function handleExportEvidenceJsonMessage() {
+  return {
+    success: true,
+    export: await exportVendorGetEvidenceJson()
+  };
+}
+
 async function handleGetLatestGvlUpdateStatusMessage() {
   const db = await openVendorGetDb();
   const latestSnapshot = await getLatestGvlSnapshotByVendorListVersion(db);