fix: date field appears for all recipients

Updates the signing endpoint to only apply changes to the Date field for the current signer. This is made possible through the addition of the `signedAt` column within the database.

Resolves the issue with one signer filling the date for all recipients and also ensures that the date of signing on a document won't always be todays date after each recipient has signed.

packages/prisma/migrations/20230421134018_doc_214_add_signed_at_field/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "Recipient" ADD COLUMN     "signedAt" TIMESTAMP(3);