feat: web push notifications for new chapters

- Service worker (src/service-worker.ts) handles push events and
  notification clicks, navigating to the book page on tap
- Web app manifest (manifest.webmanifest) linked in app.html
- Profile page: push notification toggle (subscribe/unsubscribe)
  using the browser Notification + PushManager API with VAPID
- API route POST/DELETE /api/push-subscription proxies to backend
- Go backend: push_subscriptions PocketBase collection storage
  methods (SavePushSubscription, DeletePushSubscription,
  ListPushSubscriptionsByBook) in storage/store.go
- handlers_push.go: GET vapid-public-key, POST/DELETE subscription
- webpush package: VAPID-signed sends via webpush-go, SendToBook
  fans out to all users who have the book in their library
- Runner fires push to subscribers whenever ChaptersScraped > 0
  after a successful book scrape
- Config: VAPID_PUBLIC_KEY, VAPID_PRIVATE_KEY, VAPID_SUBJECT env vars
- domain.ScrapeResult gets a Slug field; orchestrator populates it

backend/cmd/runner/main.go

@@ -34,6 +34,7 @@ import (
 	"github.com/libnovel/backend/internal/runner"
 	"github.com/libnovel/backend/internal/storage"
 	"github.com/libnovel/backend/internal/taskqueue"
+	"github.com/libnovel/backend/internal/webpush"
 )
 
 // version and commit are set at build time via -ldflags.
@@ -190,6 +191,15 @@ func run() error {
 		log.Info("runner: poll mode — using PocketBase for task dispatch")
 	}
 
+	// ── Web Push ─────────────────────────────────────────────────────────────
+	var pushSender *webpush.Sender
+	if cfg.VAPID.PublicKey != "" && cfg.VAPID.PrivateKey != "" {
+		pushSender = webpush.New(cfg.VAPID.PublicKey, cfg.VAPID.PrivateKey, cfg.VAPID.Subject, log)
+		log.Info("runner: web push notifications enabled")
+	} else {
+		log.Info("runner: VAPID_PUBLIC_KEY/VAPID_PRIVATE_KEY not set — push notifications disabled")
+	}
+
 	deps := runner.Dependencies{
 		Consumer:         consumer,
 		BookWriter:         store,
@@ -207,6 +217,8 @@ func run() error {
 		CFAI:               cfaiClient,
 		LibreTranslate:     ltClient,
 		Notifier:           store,
+		WebPush:            pushSender,
+		Store:              store,
 		Log:                log,
 	}
 	r := runner.New(rCfg, deps)