🔧(yhub) align the yhub db credentials with the backend's dev ones

The yhub database ran under its own yhub role. Every other database in
this stack uses the backend's dev credentials, dinum/pass, which is also
what CI already uses for this database; only the database name stays
apart, since it is a separate database on a separate server.

The url the collaboration server connects with moves into the same file,
beside the credentials it repeats, written once rather than twice — the
yhub service reads the file as well. A yhub-postgres.local override is
created like for the other env files, and the healthcheck asks about
POSTGRES_DB and POSTGRES_USER like the other two do instead of naming the
role.

POSTGRES_USER only takes effect on a fresh initdb, so an existing stack
needs its yhub-pgdata volume dropped and `make migrate-yhub` re-run; the
volume holds local document state only.

Signed-off-by: Kevin Jahns <kevin.jahns@protonmail.com>

src/yhub-server/README.md

@@ -596,9 +596,11 @@ provider answers to), `LEGACY_S3_SIGNATURE_VERSION` (see below), and
 `LEGACY_S3_BUCKET_NAME` (defaults to Django's dev default
 `impress-media-storage`; production uses a different bucket name and must set
 it explicitly). The server refuses to boot when the flag is set without
-endpoint and credentials. In development they come, like everything else this
-server reads, from `env.d/development/yhub` (and `yhub.local`, which is not
-committed — `make create-env-local-files` creates it).
+endpoint and credentials. In development they come from
+`env.d/development/yhub`, which holds everything this server reads but the
+database url — that one is in `env.d/development/yhub-postgres`, beside the
+credentials it spells out. Both have a `.local` counterpart, not committed,
+that `make create-env-local-files` creates.
 
 The bucket is read with the **AWS SDK for JavaScript v3**
 (`@aws-sdk/client-s3`), the same library family boto3 is to Django, so the