Tag write-path correctness: regenerate-AI bypass, upsert race, trim, transactionality

[joestump]

Part of #303 (EPIC: Unified Tag Taxonomy Completion). Governing: ADR-0025, SPEC unified-tag-taxonomy REQs "Enricher Integration", "Tag Normalization", "Denormalized Entity Tags Table". Complements #309 (dual-write removal — this story must land first).

Requirements

• Regenerate-AI bypass (review CONFIRMED + audit CRITICAL): all three regenerate handlers persist only the deprecated ai_tags JSON and discard data.TypedTags, never calling tags.UpsertTagsForEntity — regenerated tags are invisible to the taxonomy until the next background cycle (internal/handlers/tracks.go:343-375, albums.go:406-438, artists.go:548-585; metadata.go:741/1158/1453 shows the correct calls). Extract a shared persistence helper used by both paths
• Upsert race (review CONFIRMED): query-then-create against the Tag unique index — concurrent enrichment goroutines make the loser's Create fail the whole upsert (internal/tags/upsert.go:40-57). Use dialect-appropriate conflict handling or a not-found→create→conflict→re-query retry
• Trim stored display name per the normalization REQ (" shoegaze " → name "shoegaze"); currently stored verbatim (upsert.go:51) — audit WARNING
• Same-transaction semantics: the Ent edge write and the raw entity_tags INSERT run on separate connections; a crash between them desyncs the stores, and no path deletes entity_tags rows on edge removal (upsert.go:63-81) — implement or explicitly amend the spec REQ (state which in the PR)

Acceptance Criteria

• Regenerating AI tags from any show page immediately reflects in tag browsing/filtering
• Concurrent upserts of the same (name, type, user) tag both succeed (race test)

Branch

feature/349-tag-write-path-correctness

PR Convention

PR title references the story; body ends with Closes #349, references the parent epic and governing artifacts above.

🤖 Posted on behalf of @joestump by Claude.