Phase 6: journal-milter + send-log status tailer + retention

Implement the structured send log (spec 7.3), the project's highest-risk
component since a milter bug can break the relay itself.

- internal/milter: go-milter v0.4.1 journal-milter. Per-connection session
  collects SASL login, From, recipients and Subject across callbacks and
  writes one send_log "queued" row per (queue-id, recipient) at EOM
  (spec 7.3.3). Monitoring only: callbacks return Continue/Accept, recorder
  errors are logged never propagated, so it can never block mail.
- internal/logtail: polling mail.log tailer with rotation handling (inode
  change / truncation), parses sent/deferred/bounced/expired by queue-id +
  recipient and advances rows; background retention sweep prunes rows past
  SEND_LOG_RETENTION_DAYS (default 90) at startup and every 6h.
- internal/store/sendlog.go: InsertQueued, UpdateStatus (case-insensitive
  recipient match), DeleteSendLogBefore + status constants.
- cmd/panel: open the store once and share it across http/milter/tailer;
  replace the journal/logtail stubs with the real roles.
- build/postfix-config.sh: bounded milter timeouts (15/15/30s) so a hung
  milter also fails open in seconds, not the 300s default.

Fix found in-container: SASL login (app_login) was empty because go-milter
keys macros exactly as Postfix sends them, and multi-character macro names
arrive brace-wrapped ({auth_authen}); the SASL-less Phase 0 spike could not
observe this. Added a brace-tolerant macro lookup.

Verified on selfpost.example.com: gofmt/vet/unit tests green; container e2e
records rows with correct fields and advances status via the tailer; fail-open
confirmed for both an unreachable and a hung milter; retention prunes at start.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

go.mod

@@ -3,12 +3,14 @@ module codeberg.org/mix/selfpost
 go 1.26
 
 require (
+	github.com/emersion/go-milter v0.4.1
 	golang.org/x/crypto v0.54.0
 	modernc.org/sqlite v1.53.0
 )
 
 require (
 	github.com/dustin/go-humanize v1.0.1 // indirect
+	github.com/emersion/go-message v0.18.1 // indirect
 	github.com/google/uuid v1.6.0 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
 	github.com/ncruces/go-strftime v1.0.0 // indirect