Terminally fail retrying deliveries with a non-retry target type (closes #82)

Restart recovery and the 60s retry sweep both looked an orphaned
`retrying` delivery's target up in the registry and silently returned
when it did not implement `rescheduler`. If a target's type was edited
from a retry type (`http`/`slack`) to a fire-and-forget type
(`database`/`log`) or an unknown one while a delivery was still
retrying, that delivery stayed `retrying` forever.

Both sites now hand the delivery to one shared helper,
`failUnretryableRetry`, which records a `DeliveryResult` naming the
current target type as the reason and marks the delivery `failed`. It
logs at warn, not error: this is operator-caused state, not a system
fault.

Re-dispatching under the new type was rejected as it would perform a
delivery the operator never asked for; the event itself stays in the
per-webhook event database, so manual redelivery can recover it
deliberately.

Fire-and-forget targets never set status `retrying` under normal
operation, so this path stays unreachable for them in practice.

internal/delivery/engine.go

@@ -453,8 +453,9 @@ func (e *Engine) recoverRetryingDeliveries(
 // recoverSingleRetry hands an orphaned retrying delivery back
 // to its target to recompute the remaining backoff, then
 // reschedules it. Targets that do not own durable retries
-// (fire-and-forget) never produce retrying deliveries, so
-// they are skipped.
+// (fire-and-forget) never produce retrying deliveries, so a
+// delivery found in that state has had its target's type
+// changed underneath it and is terminally failed.
 func (e *Engine) recoverSingleRetry(
 	webhookDB *gorm.DB,
 	webhookID string,
@@ -475,6 +476,10 @@ func (e *Engine) recoverSingleRetry(
 
 	rs, ok := e.targets[target.Type].(rescheduler)
 	if !ok {
+		e.failUnretryableRetry(
+			webhookDB, webhookID, d, &target,
+		)
+
 		return
 	}
 
@@ -649,8 +654,8 @@ func (e *Engine) sweepWebhookRetries(
 
 // sweepSingleRetry re-enqueues an orphaned retrying delivery
 // whose backoff window has elapsed, delegating the backoff
-// decision to the delivery's target. Targets that do not own
-// durable retries are skipped.
+// decision to the delivery's target. A delivery whose target
+// no longer owns durable retries is terminally failed.
 func (e *Engine) sweepSingleRetry(
 	webhookDB *gorm.DB,
 	webhookID string,
@@ -670,6 +675,10 @@ func (e *Engine) sweepSingleRetry(
 
 	rs, ok := e.targets[target.Type].(rescheduler)
 	if !ok {
+		e.failUnretryableRetry(
+			webhookDB, webhookID, d, &target,
+		)
+
 		return
 	}
 
@@ -710,6 +719,59 @@ func (e *Engine) sweepSingleRetry(
 	}
 }
 
+// failUnretryableRetry terminally fails an orphaned retrying
+// delivery whose target type no longer supports retries. Both
+// restart recovery and the periodic sweep call it, so the
+// terminal transition exists once.
+//
+// This is only reachable when a target's type has been changed
+// out from under an in-flight retrying delivery (or the type is
+// unknown to the registry): fire-and-forget targets never set
+// status retrying themselves. Re-dispatching under the new type
+// would be a delivery the operator never asked for, and leaving
+// the row retrying strands it forever, so the delivery is
+// failed with a recorded reason and can be redelivered
+// manually. Logged at warn, not error: this is operator-caused
+// state, not a system fault.
+func (e *Engine) failUnretryableRetry(
+	webhookDB *gorm.DB,
+	webhookID string,
+	d *database.Delivery,
+	target *database.Target,
+) {
+	e.log.Warn(
+		"failing orphaned retrying delivery: target "+
+			"type no longer supports retries",
+		"webhook_id", webhookID,
+		"delivery_id", d.ID,
+		"target_id", target.ID,
+		"target_name", target.Name,
+		"target_type", target.Type,
+	)
+
+	reason := fmt.Sprintf(
+		"target type %q does not support retries; "+
+			"delivery was left retrying by a previous "+
+			"target type and has been failed terminally",
+		target.Type,
+	)
+
+	e.recordResult(
+		webhookDB,
+		d,
+		e.countAttempts(webhookDB, d.ID)+1,
+		false,
+		0,
+		"",
+		reason,
+		0,
+	)
+
+	e.updateDeliveryStatus(
+		webhookDB, d, database.DeliveryStatusFailed,
+	)
+}
+
 // processDelivery dispatches a delivery to the target that
 // owns its type. Unknown target types fail the delivery.
 func (e *Engine) processDelivery(