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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>