fix: truncate image descriptions (#7699)

* move truncate function to separate file to reuse it

* truncate image descriptions

* show image description limit in UI

* correctly treat null

Co-authored-by: nullobsi <me@nullob.si>

* make truncate Unicode-aware

The strings that truncate returns should now be valid Unicode.

PostgreSQL also counts Unicode Code Points instead of bytes so this
should be correct.

* move truncate to internal, validate in API

Truncating could also be done in src/services/drive/add-file.ts or
src/services/drive/upload-from-url.ts but those would also affect
local images. But local images should result in a hard error if the
image comment is too long.

* avoid overwriting

Co-authored-by: nullobsi <me@nullob.si>