feat: fruit overview with thumbnails and card layout (story #07)

- Auto-crop thumbnails on image upload (Go stdlib, no external deps):
  find non-background bounding box, pad 5px, scale to ≤300px, encode JPEG
- Add thumbnail_data BYTEA column to fruit_images and publication_fruit_images
- New GET /api/v1/fruits/:id/images/:imageId/thumbnail endpoint (fallback to full data)
- New GET /api/v1/publications/:id/fruit-images/:imgId/thumbnail endpoint
- New POST /api/v1/admin/backfill-thumbnails to retroactively generate thumbnails
- ListFruits response now includes synonyms and thumbnail_url per fruit
- Replace FruitList table with responsive FruitCard grid (thumbnail + name + OSDB ID + type + synonyms)

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

backend/internal/handler/publication_handler_test.go

@@ -265,6 +265,14 @@ func (r *fakePubRepo) GetFruitImageData(_ context.Context, pubID, imgID int) ([]
 	return r.fruitImgData[imgID], nil
 }
 
+func (r *fakePubRepo) GetFruitImageThumbnailData(_ context.Context, pubID, imgID int) ([]byte, error) {
+	img, ok := r.fruitImages[imgID]
+	if !ok || img.PublicationID != pubID {
+		return nil, handler.ErrNotFound
+	}
+	return r.fruitImgData[imgID], nil
+}
+
 func (r *fakePubRepo) DeleteFruitImage(_ context.Context, pubID, imgID int) error {
 	img, ok := r.fruitImages[imgID]
 	if !ok || img.PublicationID != pubID {