Default worker role to postgres instead of azuresu

[pinodeca]

Summary

The pg_durable.worker_role GUC defaulted to azuresu, the privileged admin role on Azure Database for PostgreSQL — Flexible Server. On any non-Azure PostgreSQL (vanilla, RDS, Cloud SQL, self-hosted) that role does not exist, so out of the box the background worker emitted "role does not exist" warnings and silently failed to process workflows until an operator overrode the GUC.

This changes the default to the vendor-neutral postgres role, which matches the Dockerfile and upgrade-test configuration and works on most setups.

Changes

• src/lib.rs — WORKER_ROLE GUC default and the SQL install-script fallback now use postgres
• src/types.rs — get_worker_role() fallback and its doc comment updated
• Docs that cite the default value updated: README.md, docs/rls.md, docs/security-review/workbook-data.md, docs/security-review/threat-model.dfd-lite.yaml

All three runtime fallbacks (GUC default, SQL install fallback, get_worker_role()) are kept in sync.

Notes

• Behavior change only, not a schema change — the worker role is read at runtime, not stored. The Dockerfile and test-upgrade.sh already pin postgres, so they are unaffected.
• sql/pg_durable--0.1.1.sql is intentionally left unchanged (historical version fixture for upgrade testing).
• No Azure-specific operator note was added, since only operators set this GUC.

Validation

• cargo build --features pg17 succeeds
• cargo fmt -p pg_durable -- --check passes

[tjgreen42]

LGTM