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/imaging/crop.go

@@ -0,0 +1,137 @@
+package imaging
+
+import (
+	"bytes"
+	"image"
+	"image/color"
+	"image/draw"
+	"image/jpeg"
+	_ "image/png"
+)
+
+const (
+	bgThreshold = 10
+	padding     = 5
+	maxDim      = 300
+)
+
+// CropToContent auto-crops src to the bounding box of non-background pixels,
+// adds padding, scales to fit within maxDim×maxDim, and re-encodes as JPEG.
+// Returns the original bytes unchanged if decoding fails.
+func CropToContent(src []byte) ([]byte, error) {
+	img, _, err := image.Decode(bytes.NewReader(src))
+	if err != nil {
+		return src, nil
+	}
+
+	bounds := img.Bounds()
+	if bounds.Dx() == 0 || bounds.Dy() == 0 {
+		return src, nil
+	}
+
+	bgColor := img.At(bounds.Min.X, bounds.Min.Y)
+
+	minX, minY, maxX, maxY := findContentBounds(img, bgColor, bounds)
+	if minX > maxX || minY > maxY {
+		return src, nil
+	}
+
+	minX = max(bounds.Min.X, minX-padding)
+	minY = max(bounds.Min.Y, minY-padding)
+	maxX = min(bounds.Max.X-1, maxX+padding)
+	maxY = min(bounds.Max.Y-1, maxY+padding)
+
+	cropRect := image.Rect(minX, minY, maxX+1, maxY+1)
+	cropped := cropImage(img, cropRect)
+	scaled := scaleDown(cropped)
+
+	var buf bytes.Buffer
+	if err := jpeg.Encode(&buf, scaled, &jpeg.Options{Quality: 85}); err != nil {
+		return src, nil
+	}
+	return buf.Bytes(), nil
+}
+
+func findContentBounds(img image.Image, bg color.Color, bounds image.Rectangle) (minX, minY, maxX, maxY int) {
+	minX = bounds.Max.X
+	minY = bounds.Max.Y
+	maxX = bounds.Min.X - 1
+	maxY = bounds.Min.Y - 1
+
+	for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
+		for x := bounds.Min.X; x < bounds.Max.X; x++ {
+			if channelDiff(img.At(x, y), bg) > bgThreshold {
+				if x < minX {
+					minX = x
+				}
+				if y < minY {
+					minY = y
+				}
+				if x > maxX {
+					maxX = x
+				}
+				if y > maxY {
+					maxY = y
+				}
+			}
+		}
+	}
+	return
+}
+
+func channelDiff(c1, c2 color.Color) int {
+	r1, g1, b1, _ := c1.RGBA()
+	r2, g2, b2, _ := c2.RGBA()
+	dr := abs(int(r1>>8) - int(r2>>8))
+	dg := abs(int(g1>>8) - int(g2>>8))
+	db := abs(int(b1>>8) - int(b2>>8))
+	return max(dr, max(dg, db))
+}
+
+func abs(x int) int {
+	if x < 0 {
+		return -x
+	}
+	return x
+}
+
+type subImager interface {
+	SubImage(r image.Rectangle) image.Image
+}
+
+func cropImage(img image.Image, rect image.Rectangle) image.Image {
+	if si, ok := img.(subImager); ok {
+		return si.SubImage(rect)
+	}
+	dst := image.NewRGBA(image.Rect(0, 0, rect.Dx(), rect.Dy()))
+	draw.Draw(dst, dst.Bounds(), img, rect.Min, draw.Src)
+	return dst
+}
+
+func scaleDown(img image.Image) image.Image {
+	bounds := img.Bounds()
+	w, h := bounds.Dx(), bounds.Dy()
+	if w <= maxDim && h <= maxDim {
+		return img
+	}
+
+	scaleW := float64(maxDim) / float64(w)
+	scaleH := float64(maxDim) / float64(h)
+	scale := scaleW
+	if scaleH < scale {
+		scale = scaleH
+	}
+
+	newW := max(1, int(float64(w)*scale))
+	newH := max(1, int(float64(h)*scale))
+
+	dst := image.NewRGBA(image.Rect(0, 0, newW, newH))
+	for y := 0; y < newH; y++ {
+		for x := 0; x < newW; x++ {
+			srcX := bounds.Min.X + int(float64(x)/scale)
+			srcY := bounds.Min.Y + int(float64(y)/scale)
+			dst.Set(x, y, img.At(srcX, srcY))
+		}
+	}
+	return dst
+}