fix(plugins): store task plugin source as longtext on MySQL and raise the upload cap to 8 MiB

A bare TEXT column holds 64 KiB on MySQL, and the shipped alibaba plugin
is already 73 KB, so uploading an override of it failed with "Data too
long for column 'source'" or was silently truncated. The 1 MiB upload
cap also blocks the larger plugins planned next.

Add model.LongText, a string type that picks longtext on MySQL and text
on PostgreSQL and SQLite through GormDBDataType, and use it for source
and icon. Neither type carries a length, so AutoMigrate leaves an
up-to-date column alone and widens the shipped MySQL text column once
without data loss. Raise the source cap to 8 MiB on the backend and in
the frontend picker, importers and locale strings.

Verified fresh and rc.39-upgrade startups twice each on SQLite, MySQL
5.7.44 and 8.4.11, PostgreSQL 9.6.24 and 17.11; the second start issues
no DDL and a 73 KB source round-trips on every engine.