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/admin_handler.go

@@ -0,0 +1,44 @@
+package handler
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/labstack/echo/v4"
+)
+
+type FruitBackfiller interface {
+	BackfillThumbnails(ctx context.Context) (int, error)
+}
+
+type PubBackfiller interface {
+	BackfillThumbnails(ctx context.Context) (int, error)
+}
+
+type AdminHandler struct {
+	fruitRepo FruitBackfiller
+	pubRepo   PubBackfiller
+}
+
+func NewAdminHandler(fruitRepo FruitBackfiller, pubRepo PubBackfiller) *AdminHandler {
+	return &AdminHandler{fruitRepo: fruitRepo, pubRepo: pubRepo}
+}
+
+func (h *AdminHandler) BackfillThumbnails(c echo.Context) error {
+	ctx := c.Request().Context()
+
+	fruitCount, err := h.fruitRepo.BackfillThumbnails(ctx)
+	if err != nil {
+		return c.JSON(http.StatusInternalServerError, map[string]string{"error": "backfill failed: " + err.Error()})
+	}
+
+	pubCount, err := h.pubRepo.BackfillThumbnails(ctx)
+	if err != nil {
+		return c.JSON(http.StatusInternalServerError, map[string]string{"error": "backfill failed: " + err.Error()})
+	}
+
+	return c.JSON(http.StatusOK, map[string]int{
+		"fruit_images_processed":       fruitCount,
+		"pub_fruit_images_processed":   pubCount,
+	})
+}