DLP at DATA: block, warn and override over SMTP and JMAP

Phase 2f of the DLP and mail flow rules spec: the rules now run on mail
an authenticated sender submits, after the DATA system script and
before headers and DKIM signing (§2.1).

- smtp/inbound/mailflow.rs: builds what the rules look at from the
  message (subject, the text version of each body, one level of attached
  messages, attachment text via the extractor, 10 MB of text at most)
  and the envelope (sender's groups and tenant; each recipient local or
  not, and its groups). Skipped entirely when no enabled rule applies to
  outgoing mail. Rules that can't be loaded refuse with a 451: nothing
  unchecked leaves.
- Block: 550 5.7.1 with the rule's notice. Warn: 550 5.7.1 with the
  notice and how to override: "[override: reason]" at the start of the
  subject, taken out before the message goes on (settled answer 1).
  Until phase 3, a hold rule blocks rather than let mail through.
- JMAP: EmailSubmission takes inbuxa:dlpOverride {reason}; a refusal
  comes back as inbuxa:dlpWarning or inbuxa:dlpBlocked with each rule's
  name and notice (description too, for older clients).
- Audit: one record per DLP match, the sender as actor, action create,
  target a message: the recipient domains, each rule with its detectors'
  counts, the outcome, an override's reason. Never the matched text. No
  new audit action: an older node that meets one fails its daily
  clean-up, which would make rolling back unsafe (spec §2.7 updated).

Tests: mail_rules_tests gains the DLP flow over JMAP (no rules, warning
with rule and notice, local recipient not warned, override with a
reason, block that no reason passes, the subject tag stripped from the
delivered message, audit records with no card or key text). smtp
inbound tests pass; system_tests passed twice after one timeout in the
email delivery tests that didn't recur.

crates/features/src/mailflow/engine.rs

@@ -46,7 +46,7 @@ pub struct Recipient<'a> {
 pub struct Attachment<'a> {
     pub name: Option<&'a str>,
     /// Declared type, or detected where the caller knows better.
-    pub content_type: &'a str,
+    pub content_type: Cow<'a, str>,
     pub size: u64,
     pub extracted: Extracted,
 }
@@ -606,13 +606,15 @@ mod tests {
             attachments: vec![
                 Attachment {
                     name: Some("plan.docx"),
-                    content_type: "application/vnd.openxmlformats-officedocument.wordprocessingml.document",
+                    content_type:
+                        "application/vnd.openxmlformats-officedocument.wordprocessingml.document"
+                            .into(),
                     size: 40_000,
                     extracted: Extracted::Text("IBAN GB29 NWBK 6016 1331 9268 19".into()),
                 },
                 Attachment {
                     name: Some("scan.pdf"),
-                    content_type: "application/pdf",
+                    content_type: "application/pdf".into(),
                     size: 900_000,
                     extracted: Extracted::NotInspectable(Why::Pdf),
                 },