Create unique constraint for living Tuples in Postgres datastore

[jzelinskie]

The native CockroachDB schema guarantees uniqueness of Tuples, while the Postgres datastore does not. Theoretically, it's possible for SpiceDB to create duplicate living Tuples if there is a bug. By creating a unique constraint, we can enforce only a single living Tuple for a relationship at the database level.

[ecordell]

@jzelinskie This looks done to me: https://github.com/authzed/spicedb/blob/main/internal/datastore/postgres/migrations/add_unique_living_ns.go#L5

But that commit is from before this issue was made, so maybe I'm missing something. Is there anything else we need to do here?

[jakedt]

As far as I can tell this was never an issue:
785d123#diff-14c374134210d368573327dd7ab593d8da51fe6eee637ed450f92383414d43b0R27