perf(plugins): sync task plugins from hashes and load source only for changed rows

Every node polls the active override set every 30 seconds, and the
snapshot query selected whole rows, so each poll pulled every plugin's
full source and icon from the database only to compare source_hash and
discard them. With sources allowed up to 8 MiB that is real traffic.

Skip the source and icon columns in the sync snapshot and fetch source by
row id only for plugins whose hash changed. A row never changes source in
place, so the id from the snapshot identifies exactly the text its hash
describes. A failed source load is handled like a compile failure: the
running plugin stays and the next poll retries.