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/terminal_state_test.go

@@ -178,7 +178,7 @@ func TestRecoverSingleRetry_TargetDeleted(t *testing.T) {
 	)
 
 	s.Engine.ExportRecoverWebhookDeliveries(
-		context.Background(), s.WebhookID,
+		context.Background(), s.WebhookID, atStart,
 	)
 
 	iAssertStatus(
@@ -218,11 +218,11 @@ func TestSweepSingleRetry_TargetDeleted(t *testing.T) {
 	// Twice, because the bug was an error the sweep repeated every
 	// minute for the life of the database: the second sweep must
 	// find nothing left to do.
-	s.Engine.ExportSweepWebhookRetries(
-		context.Background(), s.WebhookID,
+	s.Engine.ExportRecoverWebhookDeliveries(
+		context.Background(), s.WebhookID, onTick,
 	)
-	s.Engine.ExportSweepWebhookRetries(
-		context.Background(), s.WebhookID,
+	s.Engine.ExportRecoverWebhookDeliveries(
+		context.Background(), s.WebhookID, onTick,
 	)
 
 	iAssertStatus(
@@ -264,8 +264,8 @@ func TestSweepSingleRetry_TargetNeverExisted(t *testing.T) {
 
 	iSeedFailedResult(t, s.WebhookDB, d.ID)
 
-	s.Engine.ExportSweepWebhookRetries(
-		context.Background(), s.WebhookID,
+	s.Engine.ExportRecoverWebhookDeliveries(
+		context.Background(), s.WebhookID, onTick,
 	)
 
 	iAssertStatus(
@@ -303,8 +303,8 @@ func TestFailMissingTarget_WritesNoTargetRow(
 		database.DeliveryStatusRetrying,
 	)
 
-	s.Engine.ExportSweepWebhookRetries(
-		context.Background(), s.WebhookID,
+	s.Engine.ExportRecoverWebhookDeliveries(
+		context.Background(), s.WebhookID, onTick,
 	)
 
 	iAssertStatus(
@@ -546,7 +546,7 @@ func TestRecoverSingleRetry_TargetUnreadable_LeavesDeliveryAlone(
 	require.NoError(t, sqlDB.Close())
 
 	s.Engine.ExportRecoverRetryingDeliveries(
-		s.WebhookDB, s.WebhookID,
+		context.Background(), s.WebhookDB, s.WebhookID,
 	)
 
 	iAssertStatus(
@@ -577,7 +577,7 @@ func TestRecoverPending_TargetDeleted(t *testing.T) {
 	)
 
 	s.Engine.ExportRecoverWebhookDeliveries(
-		context.Background(), s.WebhookID,
+		context.Background(), s.WebhookID, atStart,
 	)
 
 	iAssertStatus(
@@ -620,7 +620,7 @@ func TestRecoverPending_TargetDeleted_LeavesAnOwnedDeliveryAlone(
 	require.True(t, s.Engine.ExportRetainDelivery(deliveryID))
 
 	s.Engine.ExportRecoverWebhookDeliveries(
-		context.Background(), s.WebhookID,
+		context.Background(), s.WebhookID, atStart,
 	)
 
 	iAssertStatus(
@@ -703,7 +703,7 @@ func TestSweepPending_TargetDeleted(t *testing.T) {
 
 	ctx := context.Background()
 
-	s.Engine.ExportSweepWebhookRetries(ctx, s.WebhookID)
+	s.Engine.ExportRecoverWebhookDeliveries(ctx, s.WebhookID, onTick)
 
 	tasks := fDrain(s.Engine)
 	require.Len(t, tasks, 1,
@@ -711,7 +711,7 @@ func TestSweepPending_TargetDeleted(t *testing.T) {
 	)
 	assert.Equal(t, healthy.ID, tasks[0].DeliveryID)
 
-	s.Engine.ExportSweepWebhookRetries(ctx, s.WebhookID)
+	s.Engine.ExportRecoverWebhookDeliveries(ctx, s.WebhookID, onTick)
 
 	assert.Empty(t, fDrain(s.Engine),
 		"the second sweep queued a delivery again",
@@ -807,7 +807,7 @@ func TestRecoverPending_TargetUnreadable_LeavesDeliveryAlone(
 	require.NoError(t, sqlDB.Close())
 
 	s.Engine.ExportRecoverPendingDeliveries(
-		context.Background(), s.WebhookDB, s.WebhookID,
+		context.Background(), s.WebhookDB, s.WebhookID, atStart,
 	)
 
 	iAssertStatus(