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/src/changes/get.rs

@@ -419,6 +419,7 @@ impl IntermediateChangesResponse {
             | MethodObject::DeletedAccount
             | MethodObject::AiLimits
             | MethodObject::LogSettings
+            | MethodObject::DlpSettings
             | MethodObject::DataInventory
             | MethodObject::InventorySnapshot
             | MethodObject::Explanation