Use one recovery routine at start and on every sweep tick (closes #542)

Start-up recovery and the 60-second sweep were two parallel sets of
functions doing the same job. recoverDeliveries now does it, run once at
start and then on every tick from one goroutine. A retrying delivery is
scheduled after what is left of its backoff, now if nothing is left, so
backoffElapsed is gone. The run at start still takes every pending
delivery and opens a webhook database whose file is missing; a tick
still takes only pending deliveries untouched for 15 minutes, at most
500, and skips a missing file. A retrying delivery is taken before its
event is read. The README describes one routine.

Model: opus-5-5

internal/delivery/recovery_durability_test.go

@@ -88,7 +88,7 @@ func TestRecoverySkipsPendingWithSuccessfulResult(
 	)
 
 	s.Engine.ExportRecoverPendingDeliveries(
-		context.Background(), s.WebhookDB, s.WebhookID,
+		context.Background(), s.WebhookDB, s.WebhookID, atStart,
 	)
 
 	select {
@@ -145,7 +145,7 @@ func TestRecoveryContinuesTheAttemptNumbering(t *testing.T) {
 	rSeedResult(t, s.WebhookDB, d.ID, 2, false)
 
 	s.Engine.ExportRecoverPendingDeliveries(
-		context.Background(), s.WebhookDB, s.WebhookID,
+		context.Background(), s.WebhookDB, s.WebhookID, atStart,
 	)
 
 	select {
@@ -183,8 +183,8 @@ func TestSweepRecoversStrandedPending(t *testing.T) {
 		database.DeliveryStatusPending,
 	)
 
-	s.Engine.ExportSweepWebhookRetries(
-		context.Background(), s.WebhookID,
+	s.Engine.ExportRecoverWebhookDeliveries(
+		context.Background(), s.WebhookID, onTick,
 	)
 
 	select {
@@ -230,7 +230,7 @@ func TestSweepClaimsAStrandedDeliveryOnlyOnce(t *testing.T) {
 
 	ctx := context.Background()
 
-	s.Engine.ExportSweepWebhookRetries(ctx, s.WebhookID)
+	s.Engine.ExportRecoverWebhookDeliveries(ctx, s.WebhookID, onTick)
 
 	select {
 	case task := <-s.Engine.ExportDeliveryCh():
@@ -246,7 +246,7 @@ func TestSweepClaimsAStrandedDeliveryOnlyOnce(t *testing.T) {
 		database.DeliveryStatusPending,
 	)
 
-	s.Engine.ExportSweepWebhookRetries(ctx, s.WebhookID)
+	s.Engine.ExportRecoverWebhookDeliveries(ctx, s.WebhookID, onTick)
 
 	select {
 	case task := <-s.Engine.ExportDeliveryCh():
@@ -258,9 +258,9 @@ func TestSweepClaimsAStrandedDeliveryOnlyOnce(t *testing.T) {
 	}
 }
 
-// TestSweepSettlesStrandedPendingWithoutResending is the sweep's own
-// version of the reconcile: a stranded delivery holding a successful
-// result is settled where it stands, and the receiver hears nothing.
+// TestSweepSettlesStrandedPendingWithoutResending is the reconcile on a
+// tick of the sweep: a stranded delivery holding a successful result is
+// settled where it stands, and the receiver hears nothing.
 func TestSweepSettlesStrandedPendingWithoutResending(
 	t *testing.T,
 ) {
@@ -280,8 +280,8 @@ func TestSweepSettlesStrandedPendingWithoutResending(
 	rSeedResult(t, s.WebhookDB, d.ID, 1, true)
 	rAgePending(t, s.WebhookDB, d.ID)
 
-	s.Engine.ExportSweepWebhookRetries(
-		context.Background(), s.WebhookID,
+	s.Engine.ExportRecoverWebhookDeliveries(
+		context.Background(), s.WebhookID, onTick,
 	)
 
 	select {
@@ -312,8 +312,9 @@ func TestSweepSettlesStrandedPendingWithoutResending(
 
 // TestFailedResultWriteLeavesDeliveryRecoverable is the rule the
 // targets now follow: a bookkeeping write that fails must not advance
-// the status, because pending and retrying are the states the sweeps
-// recover and delivered is a claim the database refused to record.
+// the status, because pending and retrying are the states the recovery
+// routine recovers and delivered is a claim the database refused to
+// record.
 func TestFailedResultWriteLeavesDeliveryRecoverable(
 	t *testing.T,
 ) {