🔧(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>

env.d/development/yhub-postgres

@@ -1,4 +1,13 @@
 # Postgresql db container configuration for the collaboration server (yhub).
+# Its own database on its own server: the backend's lives next to it and is
+# never touched from here. The credentials are the backend's dev ones, and the
+# ones CI uses for this database too — only the database name differs.
 POSTGRES_DB=yhub
-POSTGRES_USER=yhub
+POSTGRES_USER=dinum
 POSTGRES_PASSWORD=pass
+
+# What the collaboration server connects with: the credentials above spelled as
+# a url, which is the only form it takes. Read by the `yhub` service as well,
+# which is why it lives here rather than next to the rest of that server's
+# settings — the password is written once.
+POSTGRES=postgres://dinum:pass@yhub-postgres:5432/yhub