feat(recipe): compress hero image to 400px preview on save

Adds Thumbnailator-based ImageCompressor that resizes uploaded images
to a 400px-wide JPEG preview stored in hero_image_preview. The recipe
list uses the preview instead of the full image URL.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/test/java/com/recipeapp/recipe/ImageCompressorTest.java

@@ -0,0 +1,98 @@
+package com.recipeapp.recipe;
+
+import org.junit.jupiter.api.Test;
+
+import javax.imageio.ImageIO;
+import java.awt.*;
+import java.awt.image.BufferedImage;
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.util.Base64;
+
+import static org.assertj.core.api.Assertions.*;
+
+class ImageCompressorTest {
+
+    private final ImageCompressor compressor = new ImageCompressor();
+
+    @Test
+    void compressToPreview_returnsJpegDataUri() throws Exception {
+        String dataUri = makePngDataUri(800, 600);
+        String result = compressor.compressToPreview(dataUri);
+        assertThat(result).startsWith("data:image/jpeg;base64,");
+    }
+
+    @Test
+    void compressToPreview_outputIsDecodableJpeg() throws Exception {
+        String dataUri = makePngDataUri(800, 600);
+        String result = compressor.compressToPreview(dataUri);
+
+        String base64 = result.substring("data:image/jpeg;base64,".length());
+        byte[] bytes = Base64.getDecoder().decode(base64);
+        BufferedImage img = ImageIO.read(new ByteArrayInputStream(bytes));
+
+        assertThat(img).isNotNull();
+        assertThat(img.getWidth()).isLessThanOrEqualTo(400);
+    }
+
+    @Test
+    void compressToPreview_preservesAspectRatio() throws Exception {
+        String dataUri = makePngDataUri(800, 400); // 2:1 ratio
+        String result = compressor.compressToPreview(dataUri);
+
+        String base64 = result.substring("data:image/jpeg;base64,".length());
+        BufferedImage img = ImageIO.read(new ByteArrayInputStream(Base64.getDecoder().decode(base64)));
+
+        assertThat(img).isNotNull();
+        double ratio = (double) img.getWidth() / img.getHeight();
+        assertThat(ratio).isCloseTo(2.0, within(0.1));
+    }
+
+    @Test
+    void compressToPreview_doesNotUpscaleSmallImages() throws Exception {
+        String dataUri = makePngDataUri(200, 150); // smaller than 400px
+        String result = compressor.compressToPreview(dataUri);
+
+        String base64 = result.substring("data:image/jpeg;base64,".length());
+        BufferedImage img = ImageIO.read(new ByteArrayInputStream(Base64.getDecoder().decode(base64)));
+
+        assertThat(img).isNotNull();
+        assertThat(img.getWidth()).isLessThanOrEqualTo(200);
+    }
+
+    @Test
+    void compressToPreview_returnsNullForNull() {
+        assertThat(compressor.compressToPreview(null)).isNull();
+    }
+
+    @Test
+    void compressToPreview_returnsNullForBlankString() {
+        assertThat(compressor.compressToPreview("  ")).isNull();
+    }
+
+    @Test
+    void compressToPreview_returnsNullForNonDataUri() {
+        assertThat(compressor.compressToPreview("https://example.com/image.jpg")).isNull();
+    }
+
+    @Test
+    void compressToPreview_returnsNullForInvalidBase64() {
+        assertThat(compressor.compressToPreview("data:image/jpeg;base64,!!!not-valid!!!")).isNull();
+    }
+
+    // ── helpers ──
+
+    private String makePngDataUri(int width, int height) throws Exception {
+        BufferedImage img = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
+        Graphics2D g = img.createGraphics();
+        // draw gradient so PNG and JPEG both have non-trivial content
+        for (int x = 0; x < width; x++) {
+            g.setColor(new Color(x * 255 / width, (x * 128 / width + height / 2) % 256, 128));
+            g.drawLine(x, 0, x, height);
+        }
+        g.dispose();
+        ByteArrayOutputStream bos = new ByteArrayOutputStream();
+        ImageIO.write(img, "png", bos);
+        return "data:image/png;base64," + Base64.getEncoder().encodeToString(bos.toByteArray());
+    }
+}