ci(deploy): correct stale POSTGRES_HOST --env-file comment

obs.env documents POSTGRES_HOST but does not set a value, so obs-secrets.env
does not 'override' it — it is the only source. Reword the carried-over comment
to match reality. Raised in review (Tobias).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/actions/deploy-obs/action.yml

@@ -82,8 +82,9 @@ runs:
       # required keys before containers start. Catches undefined variables and
       # YAML errors in config files updated by the previous step.
       # --env-file order: obs.env first (git-tracked defaults), obs-secrets.env
-      # second (CI-written secrets). Later files win on duplicate keys, so
-      # obs-secrets.env overrides POSTGRES_HOST set in obs.env.
+      # second (CI-written secrets). Later files win on duplicate keys. POSTGRES_HOST
+      # is environment-specific and supplied only by obs-secrets.env — obs.env
+      # documents it but deliberately does not set a value.
       run: |
         docker compose \
           -f /opt/familienarchiv/docker-compose.observability.yml \