Pin the formatter, and stop format-gating generated migrations (#267)

`@biomejs/biome` was a caret range, so a release changed what CI considered
correctly formatted without anybody committing anything. Main has been red since
2.5.10 landed, and so is every branch opened against it. A formatter whose
verdict moves on its own is not a check.

Pinned to the version already in the lockfile, with the schema reference and the
nine files it wants brought into line.

`server/drizzle` is generated by drizzle-kit and now excluded. Reformatting a
migration snapshot is a diff of hundreds of lines that says nothing, on files
whose whole point is that they match what the tool wrote.