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{