Refactor delivery targets to a Target interface (closes #77)

Each target TYPE is now an implementation of a Target interface,
dispatched from a registry in processDelivery instead of a type
switch on TargetType. Every target owns its full delivery,
including durable retries.

- Target.Deliver receives the context, the per-webhook DB, the
  Delivery, the attempt Task, and a Scheduler for durable
  re-enqueue (the existing timer + retry queue). The target makes
  one attempt, records the DeliveryResult, updates DeliveryStatus,
  and — for retry targets — decides whether to retry, computes its
  own backoff, gates with its own circuit breaker, and reschedules
  via the Scheduler.
- httpTarget and slackTarget share a retry core (retry, backoff,
  circuit breaker). database and log targets are fire-and-forget.
- Slack retry/breaker is gated on MaxRetries: 0 stays
  fire-and-forget (existing Slack targets unchanged), >0 gets
  retry + backoff + breaker on the shared core.
- The engine keeps only the worker pool, queue/channels, restart
  recovery/sweep, the recordResult/updateDeliveryStatus helpers,
  and ScheduleRetry. Recovery/sweep hand each orphaned retrying
  delivery back to its target to recompute the backoff.
- The log target logs the entire inbound webhook: full body and
  headers, method, content type, and the webhook and entrypoint
  ids (supersedes the smaller log-summary work).
- Task gains EntrypointID, populated in the webhook handler, the
  recovery-task builder, and buildEventFromTask.

Behaviour is preserved: existing delivery tests pass with their
export_test wrappers re-pointed at the new targets; new pure
Deliver tests cover the log full-content output and the gated
Slack retry path.

internal/delivery/target_log.go

@@ -0,0 +1,47 @@
+package delivery
+
+import (
+	"context"
+
+	"gorm.io/gorm"
+	"sneak.berlin/go/webhooker/internal/database"
+)
+
+// logTarget is a fire-and-forget target that logs the entire
+// inbound webhook — the full request body and headers, plus
+// the method, content type, and the webhook and entrypoint
+// ids — then records a single successful attempt.
+type logTarget struct {
+	eng *Engine
+}
+
+// Deliver implements Target.
+func (t *logTarget) Deliver(
+	_ context.Context,
+	webhookDB *gorm.DB,
+	d *database.Delivery,
+	_ *Task,
+	_ Scheduler,
+) {
+	t.eng.log.Info(
+		"webhook event delivered to log target",
+		"delivery_id", d.ID,
+		"event_id", d.EventID,
+		"target_id", d.TargetID,
+		"target_name", d.Target.Name,
+		"webhook_id", d.Event.WebhookID,
+		"entrypoint_id", d.Event.EntrypointID,
+		"method", d.Event.Method,
+		"content_type", d.Event.ContentType,
+		"headers", d.Event.Headers,
+		"body", d.Event.Body,
+	)
+
+	t.eng.recordResult(
+		webhookDB, d, 1, true, 0, "", "", 0,
+	)
+
+	t.eng.updateDeliveryStatus(
+		webhookDB, d, database.DeliveryStatusDelivered,
+	)
+}