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/internal/runner/runner.go

@@ -36,7 +36,9 @@ import (
 	"github.com/libnovel/backend/internal/orchestrator"
 	"github.com/libnovel/backend/internal/pockettts"
 	"github.com/libnovel/backend/internal/scraper"
+	"github.com/libnovel/backend/internal/storage"
 	"github.com/libnovel/backend/internal/taskqueue"
+	"github.com/libnovel/backend/internal/webpush"
 	"github.com/prometheus/client_golang/prometheus"
 )
 
@@ -130,6 +132,12 @@ type Dependencies struct {
 	ChapterIngester ChapterIngester
 	// Notifier creates notifications for users.
 	Notifier Notifier
+	// WebPush sends browser push notifications to subscribed users.
+	// If nil, push notifications are disabled.
+	WebPush *webpush.Sender
+	// Store is the underlying *storage.Store; used for push subscription lookups.
+	// Only needed when WebPush is non-nil.
+	Store *storage.Store
 	// SearchIndex indexes books in Meilisearch after scraping.
 	// If nil a no-op is used.
 	SearchIndex meili.Client
@@ -531,6 +539,16 @@ func (r *Runner) runScrapeTask(ctx context.Context, task domain.ScrapeTask) {
 				fmt.Sprintf("Scraped %d chapters, skipped %d (%s)", result.ChaptersScraped, result.ChaptersSkipped, task.Kind),
 				"/admin/tasks")
 		}
+		// Send push notifications to users who have this book in their library.
+		if r.deps.WebPush != nil && r.deps.Store != nil &&
+			result.ChaptersScraped > 0 && result.Slug != "" && task.Kind != "catalogue" {
+			go r.deps.WebPush.SendToBook(context.Background(), r.deps.Store, result.Slug, webpush.Payload{
+				Title: "New chapter available",
+				Body:  fmt.Sprintf("%d new chapter(s) added", result.ChaptersScraped),
+				URL:   "/books/" + result.Slug,
+				Icon:  "/icon-192.png",
+			})
+		}
 	}
 
 	log.Info("runner: scrape task finished",