Take the database password from PGPASSWORD, not the DSN

The chart asked for a whole DSN in a Secret. Nothing writes one: the
Zalando postgres operator generates a Secret with `username` and
`password` keys and no connection string, so wiring the wrapper chart up
would have meant hand-maintaining a second copy of a password the
operator owns and rotates on a from-scratch rebuild -- which is charts#176
again, the issue miniflux closed by doing the opposite.

So the DSN becomes a plain value with no password in it, and the password
arrives as PGPASSWORD from a Secret. pgx fills in from libpq's PG*
environment variables whatever the DSN omits, exactly as miniflux's
lib/pq does. Verified rather than assumed, against a real server: a
password-less DSN connects with PGPASSWORD set, and fails with
`password authentication failed` when it is wrong, so the variable is
doing the work rather than being quietly ignored.

It also keeps the credential out of the rendered manifest and out of
`kubectl describe pod`, which a DSN-with-password does not.

charts/terdut-server/values.yaml

@@ -11,16 +11,28 @@ image:
   tag: "latest"
   pullPolicy: IfNotPresent
 
-# Postgres connection, as a DSN in an existing Secret:
-#   postgres://user:password@host:5432/terdut?sslmode=require
-#
-# The chart provisions no database. In this cluster the wrapper chart declares an
-# acid.zalan.do postgresql CR and points this at the Secret the operator writes;
-# anywhere else, any reachable Postgres will do.
+# Postgres connection. The chart provisions no database; it expects one to exist.
 database:
-  dsnSecret:
+  # Required. A DSN with no password in it:
+  #   postgres://terdut@terdut-postgres:5432/terdut?sslmode=require
+  #
+  # The password is deliberately a separate setting. pgx falls back to libpq's
+  # environment variables for anything the DSN omits, so PGPASSWORD supplies it
+  # without the credential appearing in values, in the rendered manifest, or in
+  # `kubectl describe pod`.
+  dsn: ""
+  # Where PGPASSWORD comes from. With the Zalando postgres operator this is the
+  # Secret it generates for the role — `<user>.<cluster>.credentials.postgresql.acid.zalan.do`,
+  # whose keys are `username` and `password` — so a from-scratch rebuild mints a
+  # new password and the server picks it up with nothing to keep in sync.
+  #
+  # Read at process start only: rotating the password needs a pod restart.
+  #
+  # Leave name empty only if the DSN carries its own password, which puts it in
+  # the manifest.
+  passwordSecret:
     name: ""
-    key: dsn
+    key: password
 
 service:
   type: ClusterIP