Expose delivery metrics on /metrics (closes #209)

/metrics carried only the inbound HTTP surface, so a destination
failing for an hour, a growing retry backlog and a stuck-open circuit
breaker were all invisible: the receive side stays healthy in each
case because it is.

New internal/metrics registers, on the existing default registry that
the go-http-metrics recorder and the promhttp handler already share:

- webhooker_events_received_total
- webhooker_delivery_attempts_total
- webhooker_deliveries_succeeded_total
- webhooker_deliveries_failed_total
- webhooker_delivery_retries_total
- webhooker_delivery_duration_seconds
- webhooker_deliveries_pending / _retrying
- webhooker_circuit_breakers_open

The route mounting is untouched.

Every delivery metric carries one label, target_type, whose domain is
the four target-type constants; anything outside it collapses to
"unknown" so no series can be minted from a UUID. Target ids, event
ids and entrypoint ids are deliberately not labels.

Instrumentation sits at the points every target type already passes
through: processDelivery for the attempt counter and the duration
histogram, updateDeliveryStatus for the outcome counters. The
queue-depth gauges are counted out of the per-webhook databases by a
30s sampler rather than tracked as deltas, which would need seeding at
startup and would drift on any transition that failed to persist. The
open-breaker gauge is recounted from the target's breaker registry on
every state change.

internal/delivery/export_test.go

@@ -10,6 +10,7 @@ import (
 	"go.uber.org/fx"
 	"gorm.io/gorm"
 	"sneak.berlin/go/webhooker/internal/database"
+	"sneak.berlin/go/webhooker/internal/metrics"
 )
 
 // ErrExportArchiveWriterEvicted exposes the sentinel returned by
@@ -253,6 +254,7 @@ func NewTestEngine(
 		deliveryCh: make(chan Task, deliveryChannelSize),
 		retryCh:    make(chan Task, retryChannelSize),
 		workers:    workers,
+		mx:         metrics.Default(),
 	}
 	e.initTargets(client)
 
@@ -267,6 +269,7 @@ func NewTestEngineSmallRetry(
 	e := &Engine{
 		log:     log,
 		retryCh: make(chan Task, 1),
+		mx:      metrics.Default(),
 	}
 	e.initTargets(nil)
 
@@ -289,12 +292,25 @@ func NewTestEngineWithDB(
 		deliveryCh: make(chan Task, deliveryChannelSize),
 		retryCh:    make(chan Task, retryChannelSize),
 		workers:    workers,
+		mx:         metrics.Default(),
 	}
 	e.initTargets(client)
 
 	return e
 }
 
+// ExportSetMetrics substitutes the engine's metric set, so a test can
+// assert on collectors registered on a private registry instead of
+// the process-wide ones every other test is also moving.
+func (e *Engine) ExportSetMetrics(mx *metrics.Set) {
+	e.mx = mx
+}
+
+// ExportSampleQueueDepths runs one queue depth sample synchronously.
+func (e *Engine) ExportSampleQueueDepths(ctx context.Context) {
+	e.sampleQueueDepths(ctx)
+}
+
 // NewTestCircuitBreaker creates a CircuitBreaker with
 // custom settings for testing.
 func NewTestCircuitBreaker(