A letter whose document is in more than one curated event's `documents`
set was linked by `putIfAbsent` over `eventRepository.findAll()`, whose
iteration order JPA does not guarantee — so the same letter could cluster
under a different event across re-seeds/VACUUM with no data change.
resolveLetterEventLinks now runs over a stably-ordered copy (earliest
event date, undated last, then lowest id), so the link is a deterministic
property of the data. Fixes review finding #9 (Architect-3).
Refs #850
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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