Add series.site_completed_at (epoch-ms, default 0): the durable answer
#168's predicates compute. checkOne writes it after the successful read
on the zero/non-zero transition only, so a Series still completed keeps
its original stamp (the age #170 prints is 'since the Site first said
so'), one that stopped is zeroed, one that became completed is stamped.
The due query projects the column, so the transition is a field
comparison on the snapshot checkOne already holds.
Refused, unreachable and errored reads reach nothing (AC4 is placement,
not a guard); the write sits before the no-chapter and unchanged-number
returns, so a completed page whose chapter did not change still writes.
Store failure logs and carries on — the outcome word never changes.
Series-level like Latest Chapter: a bookmark's updated_at is never
touched. #170 is the surface; nothing reads the column yet.
sulthan