The previous handlers did `find()` then `upsert()` in two round-trips:
- POST: two concurrent grants both pass the duplicate check; the
second `upsert` silently rewrites the role instead of returning
409, weakening the "409 on duplicate" contract under load.
- PATCH: a concurrent DELETE between `find` and `upsert` makes PATCH
silently re-create a row instead of returning 404, weakening the
"404 if no existing membership" contract.
Adds two repo primitives that fold the check into the write:
- `try_insert` — `INSERT ... ON CONFLICT DO NOTHING RETURNING`; None
return ⇒ already exists ⇒ 409.
- `update_role` — `UPDATE ... WHERE app_id AND user_id RETURNING`;
None return ⇒ no row ⇒ 404.
Handlers use these directly; existing `upsert` stays for test helpers
that genuinely want upsert semantics.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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