Add metrics, protected headers and pgpkeys actuator endpoint

- metrics (optional Micrometer): per-recipient outcome/policy/reason counter,
  refused counter, key lookup timer below the cache, cache size gauge
- SendReport.RecipientOutcome gains a structured PlaintextReason
- protected headers (RFC 9788, mail.pgp.protected-headers, off by default):
  real headers inside the encrypted part, outer Subject "[...]"
- encrypting keeps the caller's Message-ID
- pgpkeys endpoint (optional Actuator): inspect policy, lookup and key
  validity per address; evict one or all cached lookups
- KeyDiscoveryCustomizer and SecureMailSenderCustomizer extension points

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G71TQ5jDETVwdUJxGAwaiT

mailsender-demo/src/main/resources/application.yml

@@ -18,6 +18,7 @@ mail:
     mode: ${MAIL_PGP_MODE:BUILT_IN}            # BUILT_IN | RELAY
     default-policy: ${MAIL_PGP_DEFAULT_POLICY:OPPORTUNISTIC}
     mixed-mode: SPLIT
+    protected-headers: ${MAIL_PGP_PROTECTED_HEADERS:false}
     # recipients:
     #   "[*@partner.example]": REQUIRE
     # Readable sent copies: the sender's own PUBLIC key per From address.