DLP: how long held mail waits is a setting

inbuxa:DlpSettings (singleton, urn:inbuxa:jmap): keepHeldDays, 1 to 90,
7 by default (settled answer 5 made it a setting). sysDlpPolicyGet reads
it, sysDlpPolicyUpdate changes it, server-level, audited by the request
layer. Each held message keeps the days it was given, and the sender's
notices say that number. Privacy catalog entry; spec §2.6 updated.

mail_rules_tests: 7 by default, 0 refused, 3 set and a message held
afterwards expires 3 days after it was held, the expiry notice says 3.

crates/jmap-proto/src/references/eval.rs

@@ -64,6 +64,9 @@ impl Response<'_> {
                         GetResponseMethod::LogSettings(response) => {
                             response.eval_jptr(path, &mut results)
                         }
+                        GetResponseMethod::DlpSettings(response) => {
+                            response.eval_jptr(path, &mut results)
+                        }
                         GetResponseMethod::DataInventory(response) => {
                             response.eval_jptr(path, &mut results)
                         }