refactor: event-driven delivery engine with channel notifications and timer-based retries

Replace the polling-based delivery engine with a fully event-driven
architecture using Go channels and goroutines:

- Webhook handler notifies engine via buffered channel after creating
  delivery records, with inline event data for payloads < 16KB
- Large payloads (>= 16KB) use pointer semantics (Body *string = nil)
  and are fetched from DB on demand, keeping channel memory bounded
- Failed retry-target deliveries schedule Go timers with exponential
  backoff; timers fire into a separate retry channel when ready
- On startup, engine scans DB once to recover interrupted deliveries
  (pending processed immediately, retrying get timers for remaining
  backoff)
- DB stores delivery status for crash recovery only, not for
  inter-component communication during normal operation
- delivery.Notifier interface decouples handlers from engine; fx wires
  *Engine as Notifier

No more periodic polling. No more wasted cycles when idle.

internal/handlers/handlers_test.go

@@ -12,12 +12,18 @@ import (
 	"go.uber.org/fx/fxtest"
 	"sneak.berlin/go/webhooker/internal/config"
 	"sneak.berlin/go/webhooker/internal/database"
+	"sneak.berlin/go/webhooker/internal/delivery"
 	"sneak.berlin/go/webhooker/internal/globals"
 	"sneak.berlin/go/webhooker/internal/healthcheck"
 	"sneak.berlin/go/webhooker/internal/logger"
 	"sneak.berlin/go/webhooker/internal/session"
 )
 
+// noopNotifier is a no-op delivery.Notifier for tests.
+type noopNotifier struct{}
+
+func (n *noopNotifier) Notify(delivery.Notification) {}
+
 func TestHandleIndex(t *testing.T) {
 	var h *Handlers
 
@@ -41,6 +47,7 @@ func TestHandleIndex(t *testing.T) {
 			database.NewWebhookDBManager,
 			healthcheck.New,
 			session.New,
+			func() delivery.Notifier { return &noopNotifier{} },
 			New,
 		),
 		fx.Populate(&h),
@@ -76,6 +83,7 @@ func TestRenderTemplate(t *testing.T) {
 			database.NewWebhookDBManager,
 			healthcheck.New,
 			session.New,
+			func() delivery.Notifier { return &noopNotifier{} },
 			New,
 		),
 		fx.Populate(&h),