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/domain/fruit.go

@@ -11,15 +11,16 @@ var FruitTypeAliases = map[string][]string{
 }
 
 type Fruit struct {
-	ID          int          `json:"id"`
-	Name        string       `json:"name"`
-	OSDBNumber  string       `json:"osdb_number"`
-	Comment     *string      `json:"comment"`
-	FruitType   string       `json:"fruit_type"`
-	Synonyms    []string     `json:"synonyms"`
-	Images      []FruitImage `json:"images"`
-	CreatedAt   time.Time    `json:"created_at"`
-	UpdatedAt   time.Time    `json:"updated_at"`
+	ID           int          `json:"id"`
+	Name         string       `json:"name"`
+	OSDBNumber   string       `json:"osdb_number"`
+	Comment      *string      `json:"comment"`
+	FruitType    string       `json:"fruit_type"`
+	Synonyms     []string     `json:"synonyms"`
+	Images       []FruitImage `json:"images"`
+	ThumbnailURL *string      `json:"thumbnail_url"`
+	CreatedAt    time.Time    `json:"created_at"`
+	UpdatedAt    time.Time    `json:"updated_at"`
 }
 
 type FruitImage struct {