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/cmd/server/router.go

@@ -39,6 +39,7 @@ func New(pool *pgxpool.Pool) *echo.Echo {
 	g.GET("/:id/images", fruits.ListImages)
 	g.POST("/:id/images", fruits.UploadImage, middleware.BodyLimit("5M"))
 	g.GET("/:id/images/:imageId", fruits.ServeImage)
+	g.GET("/:id/images/:imageId/thumbnail", fruits.ServeThumbnail)
 	g.DELETE("/:id/images/:imageId", fruits.DeleteImage)
 
 	// Publications (story #04)
@@ -68,7 +69,13 @@ func New(pool *pgxpool.Pool) *echo.Echo {
 	p.GET("/:id/fruit-images", pubs.ListFruitImages)
 	p.POST("/:id/fruit-images", pubs.UploadFruitImage, middleware.BodyLimit("5M"))
 	p.GET("/:id/fruit-images/:imgId", pubs.ServeFruitImage)
+	p.GET("/:id/fruit-images/:imgId/thumbnail", pubs.ServeFruitImageThumbnail)
 	p.DELETE("/:id/fruit-images/:imgId", pubs.DeleteFruitImage)
 
+	// Admin (story #07)
+	admin := handler.NewAdminHandler(fruitRepo, pubRepo)
+	adminGroup := api.Group("/admin")
+	adminGroup.POST("/backfill-thumbnails", admin.BackfillThumbnails)
+
 	return e
 }