TimelineEntryDTO gains three nullable letter-only fields — rootTagId,
rootTagName, rootTagColor (token) — assembled in-transaction in TimelineService
(ADR-036): id + name + token only, never a serialized Tag entity. A letter's
primary tag is the root ancestor of its alphabetically-first assigned tag
(#827 Resolved Decision 3); roots are resolved through TagService in one
batched pass over the distinct primary tags (no per-letter N+1). The fields are
null for non-letter entries, untagged letters, and (color only) a colorless
root, so they are deliberately not @Schema(requiredMode = REQUIRED).
Refs #835
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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