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.go

@@ -9,6 +9,7 @@ import (
 
 	"go.uber.org/fx"
 	"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"
@@ -25,6 +26,7 @@ type HandlersParams struct {
 	WebhookDBMgr *database.WebhookDBManager
 	Healthcheck  *healthcheck.Healthcheck
 	Session      *session.Session
+	Notifier     delivery.Notifier
 }
 
 type Handlers struct {
@@ -34,6 +36,7 @@ type Handlers struct {
 	db        *database.Database
 	dbMgr     *database.WebhookDBManager
 	session   *session.Session
+	notifier  delivery.Notifier
 	templates map[string]*template.Template
 }
 
@@ -57,6 +60,7 @@ func New(lc fx.Lifecycle, params HandlersParams) (*Handlers, error) {
 	s.db = params.Database
 	s.dbMgr = params.WebhookDBMgr
 	s.session = params.Session
+	s.notifier = params.Notifier
 
 	// Parse all page templates once at startup
 	s.templates = map[string]*template.Template{

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),

internal/handlers/webhook.go

@@ -7,6 +7,7 @@ import (
 
 	"github.com/go-chi/chi"
 	"sneak.berlin/go/webhooker/internal/database"
+	"sneak.berlin/go/webhooker/internal/delivery"
 )
 
 const (
@@ -117,12 +118,12 @@ func (h *Handlers) HandleWebhook() http.HandlerFunc {
 
 		// Create delivery records for each active target
 		for i := range targets {
-			delivery := &database.Delivery{
+			dlv := &database.Delivery{
 				EventID:  event.ID,
 				TargetID: targets[i].ID,
 				Status:   database.DeliveryStatusPending,
 			}
-			if err := tx.Create(delivery).Error; err != nil {
+			if err := tx.Create(dlv).Error; err != nil {
 				tx.Rollback()
 				h.log.Error("failed to create delivery",
 					"target_id", targets[i].ID,
@@ -139,6 +140,23 @@ func (h *Handlers) HandleWebhook() http.HandlerFunc {
 			return
 		}
 
+		// Notify the delivery engine with inline event data so it can
+		// process deliveries immediately without a DB round-trip.
+		// Large bodies (>= 16KB) are left nil to keep channel memory
+		// bounded; the engine fetches them from DB on demand.
+		n := delivery.Notification{
+			WebhookID:   entrypoint.WebhookID,
+			EventID:     event.ID,
+			Method:      event.Method,
+			Headers:     event.Headers,
+			ContentType: event.ContentType,
+		}
+		bodyStr := string(body)
+		if len(body) < delivery.MaxInlineBodySize {
+			n.Body = &bodyStr
+		}
+		h.notifier.Notify(n)
+
 		h.log.Info("webhook event created",
 			"event_id", event.ID,
 			"webhook_id", entrypoint.WebhookID,