feat: add custom email headers (#260)

apps/web/prisma/migrations/20250927193506_add_email_headers/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "Email" ADD COLUMN     "headers" TEXT;

apps/web/prisma/schema.prisma

@@ -259,6 +259,7 @@ model Email {
   campaignId   String?
   contactId    String?
   inReplyToId  String?
+  headers      String?
   team         Team         @relation(fields: [teamId], references: [id], onDelete: Cascade)
   emailEvents  EmailEvent[]