feat(discover): Tinder-style book discovery + fix duplicate books

- New /discover page with swipe UI: left=skip, right=like, up=read now, down=nope
- Onboarding modal to collect genre/status preferences (persisted in localStorage)
- 3-card stack with pointer-event drag, CSS fly-out animation, 5 action buttons
- Tap card for preview modal; empty state with deck reset
- Like/read-now auto-saves book to user library
- POST /api/discover/vote + DELETE for deck reset
- Discovery vote persistence via PocketBase discovery_votes collection
- Fix duplicate books: dedup by slug in getBooksBySlugs
- Fix WriteMetadata TOCTOU race: conflict-retry on concurrent insert

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/storage/store.go

@@ -74,12 +74,24 @@ func (s *Store) WriteMetadata(ctx context.Context, meta domain.BookMeta) error {
 		"rating":         meta.Rating,
 	}
 	// Upsert via filter: if exists PATCH, otherwise POST.
+	// Use a conflict-retry pattern to handle concurrent scrapes racing to insert
+	// the same slug: if POST fails (or another concurrent writer beat us to it),
+	// re-fetch and PATCH instead.
 	existing, err := s.getBookBySlug(ctx, meta.Slug)
 	if err != nil && err != ErrNotFound {
 		return fmt.Errorf("WriteMetadata: %w", err)
 	}
 	if err == ErrNotFound {
-		return s.pb.post(ctx, "/api/collections/books/records", payload, nil)
+		postErr := s.pb.post(ctx, "/api/collections/books/records", payload, nil)
+		if postErr == nil {
+			return nil
+		}
+		// POST failed — a concurrent writer may have inserted the same slug.
+		// Re-fetch and fall through to PATCH.
+		existing, err = s.getBookBySlug(ctx, meta.Slug)
+		if err != nil {
+			return postErr // original POST error is more informative
+		}
 	}
 	return s.pb.patch(ctx, fmt.Sprintf("/api/collections/books/records/%s", existing.ID), payload)
 }