feat(account): retention TTL reaper (2-year legal hold)

Daily background reaper purges the deletion dossier past its two-year TTL: every
retained_identities row by detached_at (covering unlink/change on live accounts too),
and — for accounts tombstoned before the cutoff — the retained feedback thread plus the
dossier PII (deleted_display_name, last_login_ip). Chat is kept (a shared game artifact)
and the tombstone row stays. Started from main next to the guest reaper. Integration
test covers the cutoff boundary and the deleted-account feedback/PII purge.

backend/internal/account/retention.go

@@ -8,10 +8,15 @@ import (
 
 	"github.com/go-jet/jet/v2/postgres"
 	"github.com/google/uuid"
+	"go.uber.org/zap"
 
 	"scrabble/backend/internal/postgres/jet/backend/table"
 )
 
+// RetentionTTL bounds how long the account-deletion legal dossier is kept before the
+// reaper purges it: two years from the detach/deletion event (owner policy, 2026-07-03).
+const RetentionTTL = 2 * 365 * 24 * time.Hour
+
 // Reasons recorded on a retained_identities row: what detached the credential from its
 // account. The row is written just before the live identities row is removed, preserving
 // the legal dossier (which email/vk/tg was linked, and when) even as the identity frees
@@ -63,3 +68,87 @@ func (s *Store) StampLastLogin(ctx context.Context, accountID uuid.UUID, ip stri
 	}
 	return nil
 }
+
+// ReapExpiredRetention purges retention data whose event is older than cutoff: every
+// retained_identities row by its detached_at (covering unlink/change on live accounts as
+// well as deleted ones), plus — for accounts tombstoned before cutoff — the retained
+// feedback thread and the dossier PII (deleted_display_name, last_login_ip). Chat is kept
+// (a shared game artifact), and the tombstone account row itself stays (its no-cascade
+// foreign keys). It returns how many journal rows and feedback messages were removed.
+func (s *Store) ReapExpiredRetention(ctx context.Context, cutoff time.Time) (identities, feedback int64, err error) {
+	cut := postgres.TimestampzT(cutoff)
+	delJournal := table.RetainedIdentities.DELETE().
+		WHERE(table.RetainedIdentities.DetachedAt.LT(cut))
+	res, err := delJournal.ExecContext(ctx, s.db)
+	if err != nil {
+		return 0, 0, fmt.Errorf("account: reap retained identities: %w", err)
+	}
+	identities, _ = res.RowsAffected()
+
+	expired := postgres.SELECT(table.Accounts.AccountID).
+		FROM(table.Accounts).
+		WHERE(table.Accounts.DeletedAt.IS_NOT_NULL().AND(table.Accounts.DeletedAt.LT(cut)))
+	delFeedback := table.FeedbackMessages.DELETE().
+		WHERE(table.FeedbackMessages.AccountID.IN(expired))
+	fbRes, err := delFeedback.ExecContext(ctx, s.db)
+	if err != nil {
+		return identities, 0, fmt.Errorf("account: reap deleted feedback: %w", err)
+	}
+	feedback, _ = fbRes.RowsAffected()
+
+	clearPII := table.Accounts.UPDATE(table.Accounts.DeletedDisplayName, table.Accounts.LastLoginIP).
+		SET(postgres.NULL, postgres.NULL).
+		WHERE(
+			table.Accounts.DeletedAt.IS_NOT_NULL().
+				AND(table.Accounts.DeletedAt.LT(cut)).
+				AND(table.Accounts.DeletedDisplayName.IS_NOT_NULL().
+					OR(table.Accounts.LastLoginIP.IS_NOT_NULL())),
+		)
+	if _, err := clearPII.ExecContext(ctx, s.db); err != nil {
+		return identities, feedback, fmt.Errorf("account: clear expired dossier PII: %w", err)
+	}
+	return identities, feedback, nil
+}
+
+// RetentionReaper periodically purges expired account-deletion retention data via
+// Store.ReapExpiredRetention, mirroring GuestReaper: one background goroutine started once
+// from main.
+type RetentionReaper struct {
+	store *Store
+	ttl   time.Duration
+	clock func() time.Time
+	log   *zap.Logger
+}
+
+// NewRetentionReaper constructs a reaper purging retention data older than ttl. log may be
+// nil.
+func NewRetentionReaper(store *Store, ttl time.Duration, log *zap.Logger) *RetentionReaper {
+	if log == nil {
+		log = zap.NewNop()
+	}
+	return &RetentionReaper{
+		store: store,
+		ttl:   ttl,
+		clock: func() time.Time { return time.Now().UTC() },
+		log:   log,
+	}
+}
+
+// Run purges expired retention data on each tick until ctx is cancelled.
+func (r *RetentionReaper) Run(ctx context.Context, interval time.Duration) {
+	ticker := time.NewTicker(interval)
+	defer ticker.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-ticker.C:
+			idn, fb, err := r.store.ReapExpiredRetention(ctx, r.clock().Add(-r.ttl))
+			if err != nil {
+				r.log.Warn("retention reap failed", zap.Error(err))
+			} else if idn > 0 || fb > 0 {
+				r.log.Info("reaped expired retention", zap.Int64("identities", idn), zap.Int64("feedback", fb))
+			}
+		}
+	}
+}