MySQL caps `text` at 65,535 bytes. A single base64 image attachment is
allowed up to ~1.34MB by the chat route, so appending such a message fails
with ER_DATA_TOO_LONG under the default strict mode. SQLite and PostgreSQL
are unaffected because their text types are effectively unbounded.
`mediumtext` holds 16MB, comfortably above the 2MB request cap.
The migration keeps the parenthesised default expression that every other
text default in this folder uses, because MySQL rejects a literal default
on TEXT columns.
Thomas Camlong