pin_version sets pin_kind='manual' and pin_label on the target row.
Accepts already-pinned rows (promotes auto→manual, updates label).
Labels are capped at PIN_LABEL_MAX_LEN=500 chars; longer values raise
ValueError before any DB access.
unpin_version clears both fields, downgrading the row to rolling. Does
NOT delete — if the row is past the rolling FIFO depth, the next
autosave's prune will drop it.
