feat: LLM-generated construct items (design note ITEM_GENERATION.md)

Upload limits: make the byte ceiling configurable and stream-checked

Persistent storage: DATABASE_URL (Postgres/Supabase) support + deploy steps

Admin page v1: user roles (lab tier), password resets, failed-run requeue, verification queue

Apply PI decisions: full construct library (rights approved), anon cap 200, drop CRS

Sync platform from lab mainline: dev instance ready to deploy