Markus #3 / Felix B2 — kill the duplicated spec-chain across
findIdsForFilter and searchDocuments, and centralise the
"name string → Tag (find or create)" loop that updateDocumentTags and
applyBulkEditToDocument were each carrying their own copy of.
`buildSearchSpec` is the single source of truth for the seven-spec chain
(text + date range + sender + receiver + tags + tag-prefix + status). Both
callers do their own FTS short-circuit, then delegate.
`resolveTags` is the single source of truth for trimming, blank-skipping,
and find-or-create through TagService. Both updateDocumentTags (replace
semantics) and applyBulkEditToDocument (additive merge) consume it.
No behaviour change. All 231 backend tests still green.
Refs #225, PR #331
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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