(document): Add created ts and file format.

data/schema.sql

@@ -189,9 +189,10 @@ CREATE TABLE stamp_types (
 
 CREATE TABLE documents (
   document_id INTEGER GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
-  document_data VARCHAR NOT NULL,
+  document_data TEXT NOT NULL,
   seal INTEGER NOT NULL,
-  metadata TEXT
+  metadata TEXT,
+  created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
 );
 
 CREATE TABLE document_stamps (