feat: implement shared-album multi-user photo album

Backend (Go/Gin/pgx): env config, pgx pool with retries, Garage S3 client
(path-style, region garage), embedded golang-migrate migrations, JWT auth with
refresh rotation, albums CRUD, members/roles/invites with no-escalation matrix,
photo upload + async imaging/goexif WebP variants, cursor pagination, presigned
URLs, likes/comments, SSE hub (album + personal), notifications, share links.

Frontend (SvelteKit 2 + Svelte 5 + Tailwind v4): API wrapper with 401 refresh,
auth store, all screens, Uploader/PhotoGrid/Lightbox/Comments, SSE client with
reconnect + REST catch-up, three themes + system via data-theme.

Deploy without Docker: Makefile builds the frontend into backend/static and
embeds it (go:embed) into a single statically linked binary; systemd unit and
nginx SSE example included.

Use gen2brain/webp (-tags nodynamic) instead of cgo kolesa-team/go-webp to keep
CGO_ENABLED=0 static builds; design.md/proposal.md updated accordingly.

backend/internal/notifications/service.go

@@ -0,0 +1,44 @@
+package notifications
+
+import (
+	"context"
+
+	"shared-album/internal/auth"
+	"shared-album/internal/httpx"
+	"shared-album/internal/models"
+	"shared-album/internal/sse"
+)
+
+type Service struct {
+	store *Store
+	hub   *sse.Hub
+}
+
+func NewService(store *Store, hub *sse.Hub) *Service {
+	return &Service{store: store, hub: hub}
+}
+
+func (s *Service) Create(ctx context.Context, userID, typ string, payload map[string]any) error {
+	n := &models.Notification{
+		ID:          auth.NewID(),
+		UserID:      userID,
+		Type:        typ,
+		PayloadJSON: payload,
+	}
+	if err := s.store.Create(ctx, n); err != nil {
+		return err
+	}
+	s.hub.PublishUser(userID, "notification", n)
+	return nil
+}
+
+func (s *Service) List(ctx context.Context, userID string, limit int) ([]models.Notification, error) {
+	return s.store.List(ctx, userID, limit)
+}
+
+func (s *Service) MarkRead(ctx context.Context, id, userID string) error {
+	if err := s.store.MarkRead(ctx, id, userID); err != nil {
+		return httpx.NotFound("notification_not_found", "notification not found")
+	}
+	return nil
+}