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/orchestrator/orchestrator.go

@@ -90,6 +90,7 @@ func (o *Orchestrator) RunBook(ctx context.Context, task domain.ScrapeTask) doma
 		result.Errors++
 		return result
 	}
+	result.Slug = meta.Slug
 
 	if err := o.store.WriteMetadata(ctx, meta); err != nil {
 		o.log.Error("metadata write failed", "slug", meta.Slug, "err", err)
@@ -97,6 +98,7 @@ func (o *Orchestrator) RunBook(ctx context.Context, task domain.ScrapeTask) doma
 		result.Errors++
 	} else {
 		result.BooksFound = 1
+		result.Slug = meta.Slug
 		// Fire optional post-metadata hook (e.g. Meilisearch indexing).
 		if o.postMetadata != nil {
 			o.postMetadata(ctx, meta)