fix(db-tool): prevent fd leak from logger to long-running daemons

The hectic logger opens fd 3 as a dup of stderr. Child processes inherit
this fd, and daemonized PostgreSQL/PostgREST keeping it open prevents the
terminal from returning to the prompt after the spawning script exits.

- Add with_closed_fds helper that runs commands in a subshell with fds
  3-9 redirected to /dev/null
- Inline the helper into both database and postgres-init builds
- Wrap pg_ctl start and postgrest with the helper

legacy/helper/posix-shell/default.nix

@@ -21,4 +21,7 @@ in {
   pager_or_cat = hectic.writeDash "pager_or_cat.sh" ''
     ${builtins.readFile ./pager_or_cat.sh}
   '';
+  with_closed_fds = hectic.writeDash "with_closed_fds.sh" ''
+    ${builtins.readFile ./with_closed_fds.sh}
+  '';
 }