Harden LLM response handling; fix mixtape management bugs

[joestump]

Part of #320. Governing: ADR-0008, ADR-0017, SPEC vibes-ai-mixtape-engine, SPEC similar-artists-discovery. All review CONFIRMED.

Requirements

• Process-killing panic: callOpenAI returns resp.Choices[0].Message.Content with no length guard, and all three calling handlers spawn GenerateMixtape in bare goroutines with no recover() — an empty-choices 200 response (content filter, proxy passthrough) crashes the server (internal/vibes/generator.go:650; enhancer.go:513 already has the guard). Add the guard AND recover() in the spawned goroutines (publish a mixtape-error event on panic)
• Nil artist edge panic: getListeningHistory dereferences t.Edges.Artist.Name without the nil check Album gets; Track's artist edge is not Required in the schema (internal/vibes/enhancer.go:385,391)
• Dead cover-art prompting: EnrichAlbum sets data.HasCoverArt=true after the prompt template has already executed, so the {{.HasCoverArt}} branches in album.tmpl never fire — cover-art commentary/dominant colors silently always empty (internal/enrichers/openai/openai.go:583-607)
• Similar-artists data loss: storeSimilarArtists deletes all existing rows before inserting, no transaction — an LLM response whose artists match nothing wipes prior data and returns nil (internal/services/similar_artists.go:358-414). Validate matches first, write in a transaction
• DeleteDJ 500s forever: deleting a DJ that owns mixtapes hits the NOT NULL FK (OnDelete: NoAction, _fk=1) → constraint error → 500; DJ undeletable from the UI (internal/handlers/vibes.go:228). Cascade or block-with-message
• Invalid schedule form value → ent enum validation error → 500; validate and return 400 with a usable message (vibes.go:354,447)

Acceptance Criteria

• Empty-choices LLM response yields a user-visible mixtape-error event, process alive
• Enhancement over a listen with an artist-less track completes
• Regenerated album enrichment includes cover-art commentary when images exist
• DJ with mixtapes is deletable (or cleanly refused), never 500

Branch

feature/339-llm-hardening-mixtape-fixes

PR Convention

PR title references the story; body ends with Closes #339, references the parent epic and governing artifacts above.

🤖 Posted on behalf of @joestump by Claude.