Replay every unanimous upstream status, not just 4xx

openvoxdb does not reserve 5xx for its own faults: the same malformed
query is a 400 on /nodes and a 500 on /facts, and /metrics answers a flat
403, so a 4xx-only replay rule made pdbmux's behaviour depend on the
route. The meta and metrics handlers held their own copy of the gateway
error and bypassed the replay entirely.

- Replay any status from 400 up that every backend agreed on, with the
  backend's own body and content type.
- Keep 502 for backends disagreeing on the status, or a backend that
  answered nothing at all.
- Route /pdb/meta, /metrics and the pass-through path through the same
  rule as the merged query handlers.
- Count a unanimous 5xx as a failed round and let it fall back to a stale
  cache entry; only a unanimous 4xx stays exempt from both.
- Answer successful queries with openvoxdb's application/json;charset=utf-8.

metrics.go

@@ -65,7 +65,13 @@ func (s *Server) handleMetrics(w http.ResponseWriter, r *http.Request) {
 		writeRaw(w, alive[0])
 		return
 	}
-	w.Header().Set("Content-Type", "application/json")
+	// The merged body is the backends' own payload reshaped, so it keeps their
+	// content type rather than announcing a different one.
+	contentType := alive[0].contentType
+	if contentType == "" {
+		contentType = jsonContentType
+	}
+	w.Header().Set("Content-Type", contentType)
 	_, _ = w.Write(body)
 }