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/object/mod.rs

@@ -24,6 +24,7 @@ pub mod fastmail_masked_email; // inbuxa: masked email
 pub mod inbuxa_account_lock; // inbuxa: account lock with delegation
 pub mod inbuxa_ai_limits; // inbuxa: AI spam classification
 pub mod inbuxa_log_settings; // inbuxa: personal-data catalog, D1
+pub mod inbuxa_dlp_settings; // inbuxa: DLP settings
 pub mod inbuxa_data_inventory; // inbuxa: personal-data catalog
 pub mod inbuxa_inventory_snapshot; // inbuxa: personal-data catalog
 pub mod inbuxa_audit; // inbuxa: the audit log