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.

resources/privacy/catalog.toml

@@ -79,6 +79,10 @@ lockedAt = ["metadata"]
 lockedBy = ["identifier"]
 delegates = ["identifier"]
 
+[object."inbuxa:DlpSettings"]
+file = "inbuxa_dlp_settings.rs"
+default = "none"
+
 [object."inbuxa:HeldMessage"]
 file = "inbuxa_held_message.rs"
 default = "none"