feat(local-stack): docker-compose dev stack + helper scripts

End-to-end developer experience without any hosted dependency.

  * docker-compose.dev.yml         - Postgres 16, PostgREST 12.2.3,
                                     and Azurite (Azure Blob emulator)
                                     wired together for AUTH_PROVIDER=local
                                     development.  Plays the role our
                                     Bicep stack plays in production.

  * scripts/local-stack/00-init-roles.sql
                                   - recreates the PostgREST role
                                     topology (web_anon / authenticated
                                     / service_role) on the freshly
                                     provisioned local Postgres.  This
                                     is the same role topology that the
                                     0005_postgres_roles migration
                                     applies to non-Supabase Azure
                                     Postgres deployments.

  * scripts/local-stack/forge-jwt.mjs
                                   - mints HS256 service-role and
                                     authenticated-role JWTs against
                                     JWT_SECRET so the local backend
                                     and tests can sign in without
                                     standing up a real IdP.

  * scripts/dev-infra-check.ps1     - smoke check: validates Postgres
                                     reachability, the role topology,
                                     PostgREST schema cache, and
                                     Azurite blob endpoints.  Prints a
                                     human-readable status report so a
                                     fresh clone can confirm the stack
                                     is wired up correctly.