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/pom.xml

@@ -55,6 +55,16 @@
 			<artifactId>postgresql</artifactId>
 			<scope>runtime</scope>
 		</dependency>
+		<dependency>
+			<groupId>net.coobird</groupId>
+			<artifactId>thumbnailator</artifactId>
+			<version>0.4.21</version>
+		</dependency>
+		<dependency>
+			<groupId>com.twelvemonkeys.imageio</groupId>
+			<artifactId>imageio-webp</artifactId>
+			<version>3.13.1</version>
+		</dependency>
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-test</artifactId>