Phase 5 scalability and polish:
- Add tags + prompt_tags junction tables replacing JSON column
- Migrate existing JSON tags via json_each() on startup
- Rewrite all tag queries to use junction tables (O(1) vs O(n))
- Simplify rename/delete/merge tag operations to single SQL
- Batch-attach preview images to prompt list responses (fix N+1)
- Fix fetchone() fragility under thread contention (defensive defaults)