feat: add manual migrations using gormigration

[rolznz]

• initial migration (postgres and sqlite)
• update preimage to be NULL instead of ''
• add composite index for summing payments in budget period

also fixes preimage to be NULL by default and removes gorm model hints

Manually tested with:

• existing DB (current release schema): sqlite
• new DB: sqlite
• existing DB (current release schema): postgres
• new DB: postgres

Notes:

• initial migrations of sqlite and postgres are generated from their respective schema dumps (PostgreSQL 16, sqlite3 3.37.2)
• I used a simple method of checking if the database is empty or not to skip the first migration so that I did not need to alter all the lines of the dumps and accidentally break something
• current format of migrations seem verbose and prone to accidental copy-paste errors (ID/filename). Is there a better way?

[bumi]

need to test it locally, but looks good to me!

[kiwiidb]

Some pieces could be a bit more idiomatic.

[rolznz]

@kiwiidb all feedback has been addressed. Thanks!

[rolznz]

@bumi did you test it locally?

[kiwiidb]

@bumi did you test it locally?

I have, in all the scenario's that you also mentioned.

[bumi]

yep, I did and looks good to me.