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/config/config.go

@@ -123,6 +123,19 @@ type Redis struct {
 	Password string
 }
 
+// VAPID holds Web Push VAPID key pair for browser push notifications.
+// Generate a pair once with: go run ./cmd/genkeys (or use the web-push CLI).
+// The public key is exposed via GET /api/push-subscriptions/vapid-public-key
+// and embedded in the SvelteKit app via PUBLIC_VAPID_PUBLIC_KEY.
+type VAPID struct {
+	// PublicKey is the base64url-encoded VAPID public key (65 bytes, uncompressed EC P-256).
+	PublicKey string
+	// PrivateKey is the base64url-encoded VAPID private key (32 bytes).
+	PrivateKey string
+	// Subject is the mailto: or https: URL used as the VAPID subscriber contact.
+	Subject string
+}
+
 // Runner holds settings specific to the runner/worker binary.
 type Runner struct {
 	// PollInterval is how often the runner checks PocketBase for pending tasks.
@@ -172,6 +185,7 @@ type Config struct {
 	Meilisearch    Meilisearch
 	Valkey         Valkey
 	Redis          Redis
+	VAPID          VAPID
 	// LogLevel is one of "debug", "info", "warn", "error".
 	LogLevel string
 }
@@ -258,6 +272,12 @@ func Load() Config {
 			Addr:     envOr("REDIS_ADDR", ""),
 			Password: envOr("REDIS_PASSWORD", ""),
 		},
+
+		VAPID: VAPID{
+			PublicKey:  envOr("VAPID_PUBLIC_KEY", ""),
+			PrivateKey: envOr("VAPID_PRIVATE_KEY", ""),
+			Subject:    envOr("VAPID_SUBJECT", "mailto:admin@libnovel.cc"),
+		},
 	}
 }