Add grant runAs for Postgres grants

[MelbourneDeveloper]

Summary

• add grants[].runAs for PostgreSQL support grants and YAML round-trip support
• wrap runAs grants with a membership guard plus SET LOCAL ROLE / RESET ROLE so migration roles can apply grants owned by another schema role
• preserve grant diff idempotency and add NAP-shaped Supabase auth grant E2E coverage
• add mixed-case CHECK constraint regression coverage for the existing PostgreSQL identifier path

Fixes #46
Refs #18

Validation

• dotnet test Migration/Nimblesite.DataProvider.Migration.Tests/Nimblesite.DataProvider.Migration.Tests.csproj --configuration Release --verbosity minimal (484 passed)
• make lint