fail closed on quarantine misconfig and first-seen errors

- backfill quarantine_days<=0 to 3 with migration 0006
- reject quarantine_enabled with quarantine_days<1 on create and update
- cap upstream Last-Modified at now
- answer 503 with Retry-After when the first-seen lookup fails

internal/api/v1/proxy.go

@@ -153,6 +153,9 @@ func (h *ProxyHandler) serveFetch(w http.ResponseWriter, r *http.Request, remote
 
 // writeProxyError answers a quarantine refusal as JSON and anything else as text.
 func writeProxyError(w http.ResponseWriter, pe *proxy.ProxyError) {
+	if pe.RetryAfter > 0 {
+		w.Header().Set("Retry-After", strconv.Itoa(pe.RetryAfter))
+	}
 	if pe.Published.IsZero() {
 		http.Error(w, pe.Message, pe.Status)
 		return