delivery log: measure the sweep before deciding its fate (audit H3, 0007)

Migration 0007 (DDL only): delivery_log_days, a row per UTC day, kept a
year, shown nowhere - read off a copy of the database after a month by the
decision rule written in the audit plan.

- reconcile_closed: the sweep adds the rows it actually closed.
- reconcile_contradicted: store.UpdateStatus, in one transaction, counts the
  rows with reason "reconcile" a delivery line now speaks of, writes the
  log's status and clears the reason - so a re-read line counts once.
- tailer_blind_seconds: follow() tracks stretches in which the file is gone
  or reopening fails (logtail.blindness); stored when a stretch ends and
  every minute while it lasts; under 3 s not counted (the rotation's tick
  between rename and create loses nothing).
- queued_over_lifetime: every sweep overwrites the day's level with the rows
  queued past LongestLifetime + MaximalBackoff (a message Postfix still
  works on stays queued until its first retry after the lifetime).
- retentionLoop prunes days older than a year.

Tests: store (sums per day, the level overwritten, under a second nothing,
prune; a contradicted sweep counted once and its reason cleared; count of
queued rows); logtail (the sweep counts what it closed; countStuck records
the level; blindness - a one-tick gap not counted, 75 s stored as 60 + 15
from where the last part ended). Schema tests at user_version 7.

schema-migrations.md: chain head 7 (it still said 5 since 2.4.0), row 7.
CHANGELOG [Unreleased], audit plan line ticked, roadmap 10/61.
Evidence: go vet ./... clean; go test ./internal/store ok, ./internal/logtail
ok but for TestFollowTailsAndRotates (Windows); the server run of the stage
follows before the push.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/logtail/retention_test.go

@@ -27,6 +27,12 @@ func (s *retentionProbeStore) CloseUnreported(string, string) (int64, error) {
 	return 0, nil
 }
 
+func (s *retentionProbeStore) AddReconcileClosed(time.Time, int64) error     { return nil }
+func (s *retentionProbeStore) AddTailerBlind(time.Time, time.Duration) error { return nil }
+func (s *retentionProbeStore) CountQueuedOlderThan(time.Time) (int64, error) { return 0, nil }
+func (s *retentionProbeStore) SetQueuedOverLifetime(time.Time, int64) error  { return nil }
+func (s *retentionProbeStore) PruneDeliveryLogDays(time.Time) error          { return nil }
+
 func (s *retentionProbeStore) DeleteSendLogBefore(cutoff time.Time) (int64, error) {
 	s.mu.Lock()
 	s.cutoff = cutoff