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.

README.md

@@ -636,6 +636,18 @@ This means:
    durable fallback that ensures no retry is permanently lost, even under
    extreme backpressure.
 
+**Changing a target's type does not migrate in-flight deliveries.** Only
+`http` and `slack` targets own durable retries; `database` and `log`
+targets are fire-and-forget and never produce a `retrying` delivery. If a
+target's `type` is edited from a retrying type to a non-retrying (or
+unknown) one while one of its deliveries is still `retrying`, both
+recovery paths above terminally mark that delivery `failed` and record a
+`DeliveryResult` naming the current target type as the reason, logging it
+at warn level. The delivery is not re-dispatched under the new type — the
+operator never asked for that delivery — and the event itself remains
+stored in the per-webhook event database, so it can be redelivered
+manually.
+
 ### Circuit Breaker (HTTP Targets with Retries)
 
 HTTP targets with `max_retries` > 0 are protected by a **per-target circuit breaker** that