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/repository/publication_repo.go

@@ -10,6 +10,7 @@ import (
 
 	"osdb/internal/domain"
 	"osdb/internal/handler"
+	"osdb/internal/imaging"
 )
 
 type PublicationRepo struct {
@@ -332,12 +333,15 @@ func (r *PublicationRepo) AddFruitImage(ctx context.Context, pubID, fruitID int,
 	if err := r.requirePublication(ctx, pubID); err != nil {
 		return domain.PublicationFruitImage{}, err
 	}
+
+	thumbnailData, _ := imaging.CropToContent(data)
+
 	var img domain.PublicationFruitImage
 	err := r.pool.QueryRow(ctx,
-		`INSERT INTO publication_fruit_images (publication_id, fruit_id, filename, data, image_type)
-		 VALUES ($1, $2, $3, $4, 'fruit')
+		`INSERT INTO publication_fruit_images (publication_id, fruit_id, filename, data, thumbnail_data, image_type)
+		 VALUES ($1, $2, $3, $4, $5, 'fruit')
 		 RETURNING id, publication_id, fruit_id, filename, image_type, created_at`,
-		pubID, fruitID, filename, data).
+		pubID, fruitID, filename, data, thumbnailData).
 		Scan(&img.ID, &img.PublicationID, &img.FruitID, &img.Filename, &img.ImageType, &img.CreatedAt)
 	if err != nil {
 		return domain.PublicationFruitImage{}, err
@@ -357,6 +361,24 @@ func (r *PublicationRepo) GetFruitImageData(ctx context.Context, pubID, imgID in
 	return data, err
 }
 
+// GetFruitImageThumbnailData returns thumbnail_data if present, falls back to data.
+func (r *PublicationRepo) GetFruitImageThumbnailData(ctx context.Context, pubID, imgID int) ([]byte, error) {
+	var thumbnailData, data []byte
+	err := r.pool.QueryRow(ctx,
+		`SELECT thumbnail_data, data FROM publication_fruit_images WHERE id=$1 AND publication_id=$2`,
+		imgID, pubID).Scan(&thumbnailData, &data)
+	if errors.Is(err, pgx.ErrNoRows) {
+		return nil, handler.ErrNotFound
+	}
+	if err != nil {
+		return nil, err
+	}
+	if len(thumbnailData) > 0 {
+		return thumbnailData, nil
+	}
+	return data, nil
+}
+
 func (r *PublicationRepo) DeleteFruitImage(ctx context.Context, pubID, imgID int) error {
 	tag, err := r.pool.Exec(ctx,
 		`DELETE FROM publication_fruit_images WHERE id=$1 AND publication_id=$2`, imgID, pubID)
@@ -426,3 +448,41 @@ func (r *PublicationRepo) requirePublication(ctx context.Context, pubID int) err
 	}
 	return nil
 }
+
+// BackfillThumbnails generates thumbnail_data for all publication_fruit_images rows missing it.
+// Returns count of rows processed.
+func (r *PublicationRepo) BackfillThumbnails(ctx context.Context) (int, error) {
+	rows, err := r.pool.Query(ctx,
+		`SELECT id, data FROM publication_fruit_images WHERE thumbnail_data IS NULL`)
+	if err != nil {
+		return 0, err
+	}
+	defer rows.Close()
+
+	type row struct {
+		id   int
+		data []byte
+	}
+	var pending []row
+	for rows.Next() {
+		var rw row
+		if err := rows.Scan(&rw.id, &rw.data); err != nil {
+			return 0, err
+		}
+		pending = append(pending, rw)
+	}
+	if err := rows.Err(); err != nil {
+		return 0, err
+	}
+
+	count := 0
+	for _, rw := range pending {
+		thumb, _ := imaging.CropToContent(rw.data)
+		if _, err := r.pool.Exec(ctx,
+			`UPDATE publication_fruit_images SET thumbnail_data=$1 WHERE id=$2`, thumb, rw.id); err != nil {
+			return count, err
+		}
+		count++
+	}
+	return count, nil
+}