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/storage/store.go

@@ -1526,3 +1526,116 @@ func parseAIJob(raw json.RawMessage) (domain.AIJob, error) {
 		HeartbeatAt:  parseT(r.HeartbeatAt),
 	}, nil
 }
+
+// ── Push subscriptions ────────────────────────────────────────────────────────
+
+// PushSubscription holds the Web Push subscription data for a single browser.
+type PushSubscription struct {
+	ID       string
+	UserID   string
+	Endpoint string
+	P256DH   string
+	Auth     string
+}
+
+// SavePushSubscription upserts a Web Push subscription for a user.
+// If a record with the same endpoint already exists it is updated in place.
+func (s *Store) SavePushSubscription(ctx context.Context, sub PushSubscription) error {
+	filter := fmt.Sprintf("endpoint=%q", sub.Endpoint)
+	existing, err := s.pb.listAll(ctx, "push_subscriptions", filter, "")
+	if err != nil {
+		return fmt.Errorf("SavePushSubscription list: %w", err)
+	}
+	payload := map[string]any{
+		"user_id":  sub.UserID,
+		"endpoint": sub.Endpoint,
+		"p256dh":   sub.P256DH,
+		"auth":     sub.Auth,
+	}
+	if len(existing) > 0 {
+		var rec struct {
+			ID string `json:"id"`
+		}
+		if json.Unmarshal(existing[0], &rec) == nil && rec.ID != "" {
+			return s.pb.patch(ctx, fmt.Sprintf("/api/collections/push_subscriptions/records/%s", rec.ID), payload)
+		}
+	}
+	return s.pb.post(ctx, "/api/collections/push_subscriptions/records", payload, nil)
+}
+
+// DeletePushSubscription removes a Web Push subscription by endpoint.
+func (s *Store) DeletePushSubscription(ctx context.Context, userID, endpoint string) error {
+	filter := fmt.Sprintf("user_id=%q&&endpoint=%q", userID, endpoint)
+	items, err := s.pb.listAll(ctx, "push_subscriptions", filter, "")
+	if err != nil {
+		return fmt.Errorf("DeletePushSubscription list: %w", err)
+	}
+	for _, raw := range items {
+		var rec struct {
+			ID string `json:"id"`
+		}
+		if json.Unmarshal(raw, &rec) == nil && rec.ID != "" {
+			_ = s.pb.delete(ctx, fmt.Sprintf("/api/collections/push_subscriptions/records/%s", rec.ID))
+		}
+	}
+	return nil
+}
+
+// ListPushSubscriptionsByBook returns all push subscriptions belonging to users
+// who have the given book slug in their library (user_library collection).
+func (s *Store) ListPushSubscriptionsByBook(ctx context.Context, slug string) ([]PushSubscription, error) {
+	// Find all users who have this book in their library
+	libFilter := fmt.Sprintf("slug=%q&&user_id!=''", slug)
+	libItems, err := s.pb.listAll(ctx, "user_library", libFilter, "")
+	if err != nil {
+		return nil, fmt.Errorf("ListPushSubscriptionsByBook list library: %w", err)
+	}
+
+	// Collect unique user IDs
+	seen := make(map[string]bool)
+	var userIDs []string
+	for _, raw := range libItems {
+		var rec struct {
+			UserID string `json:"user_id"`
+		}
+		if json.Unmarshal(raw, &rec) == nil && rec.UserID != "" && !seen[rec.UserID] {
+			seen[rec.UserID] = true
+			userIDs = append(userIDs, rec.UserID)
+		}
+	}
+	if len(userIDs) == 0 {
+		return nil, nil
+	}
+
+	// Build OR filter for push_subscriptions
+	parts := make([]string, len(userIDs))
+	for i, uid := range userIDs {
+		parts[i] = fmt.Sprintf("user_id=%q", uid)
+	}
+	subFilter := strings.Join(parts, "||")
+	subItems, err := s.pb.listAll(ctx, "push_subscriptions", subFilter, "")
+	if err != nil {
+		return nil, fmt.Errorf("ListPushSubscriptionsByBook list subs: %w", err)
+	}
+
+	subs := make([]PushSubscription, 0, len(subItems))
+	for _, raw := range subItems {
+		var rec struct {
+			ID       string `json:"id"`
+			UserID   string `json:"user_id"`
+			Endpoint string `json:"endpoint"`
+			P256DH   string `json:"p256dh"`
+			Auth     string `json:"auth"`
+		}
+		if json.Unmarshal(raw, &rec) == nil && rec.Endpoint != "" {
+			subs = append(subs, PushSubscription{
+				ID:       rec.ID,
+				UserID:   rec.UserID,
+				Endpoint: rec.Endpoint,
+				P256DH:   rec.P256DH,
+				Auth:     rec.Auth,
+			})
+		}
+	}
+	return subs, nil
+}