db-platform-fit (M21) — version · pricing/lock-in · per-platform feature support

M21 audits the project against constraints imposed by the managed platform, not the raw engine. The same Postgres schema behaves differently on Neon (serverless, scale-to-zero, branching) vs RDS vs Supabase (RLS-centric, PostgREST). Findings feed both axes: feature gaps (FK support, RLS) are design; connection/cold-start/version issues are performance.