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/fruit_handler.go

@@ -29,6 +29,7 @@ type FruitRepository interface {
 	ListImages(ctx context.Context, fruitID int) ([]domain.FruitImage, error)
 	AddImage(ctx context.Context, fruitID int, filename *string, data []byte, imageType string, title *string) (domain.FruitImage, error)
 	GetImageData(ctx context.Context, fruitID, imageID int) ([]byte, error)
+	GetThumbnailData(ctx context.Context, fruitID, imageID int) ([]byte, error)
 	DeleteImage(ctx context.Context, fruitID, imageID int) error
 }
 
@@ -291,3 +292,20 @@ func (h *FruitHandler) DeleteImage(c echo.Context) error {
 	}
 	return c.NoContent(http.StatusNoContent)
 }
+
+func (h *FruitHandler) ServeThumbnail(c echo.Context) error {
+	fruitID, err := parseID(c, "id")
+	if err != nil {
+		return err
+	}
+	imageID, err := parseID(c, "imageId")
+	if err != nil {
+		return err
+	}
+	data, err := h.repo.GetThumbnailData(c.Request().Context(), fruitID, imageID)
+	if err != nil {
+		return mapRepoError(c, err)
+	}
+	contentType := http.DetectContentType(data)
+	return c.Blob(http.StatusOK, contentType, data)
+}