Add startup checks

pom.xml

@@ -107,6 +107,27 @@
                     <release>25</release>
                 </configuration>
             </plugin>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-enforcer-plugin</artifactId>
+                <version>3.5.0</version>
+                <executions>
+                    <execution>
+                        <id>require-java-25</id>
+                        <goals>
+                            <goal>enforce</goal>
+                        </goals>
+                        <configuration>
+                            <rules>
+                                <requireJavaVersion>
+                                    <version>[25,)</version>
+                                    <message>JDK 25 or newer is required. Configure IntelliJ and Maven to use JDK 25 before running the backend.</message>
+                                </requireJavaVersion>
+                            </rules>
+                        </configuration>
+                    </execution>
+                </executions>
+            </plugin>
             <plugin>
                 <groupId>org.springframework.boot</groupId>
                 <artifactId>spring-boot-maven-plugin</artifactId>

src/main/java/com/petshop/backend/BackendApplication.java

@@ -9,6 +9,7 @@ import org.springframework.data.web.config.EnableSpringDataWebSupport;
 @EnableSpringDataWebSupport(pageSerializationMode = EnableSpringDataWebSupport.PageSerializationMode.VIA_DTO)
 public class BackendApplication {
     public static void main(String[] args) {
+        RuntimeClasspathValidator.validate();
         new SpringApplicationBuilder(BackendApplication.class)
                 .initializers(new FlywayContextInitializer())
                 .run(args);

src/main/java/com/petshop/backend/DevStackApplication.java

@@ -28,6 +28,7 @@ public class DevStackApplication {
 
         try {
             validateJavaVersion();
+            RuntimeClasspathValidator.validate();
             docker.ensureDockerAvailable();
             docker.startDatabase();
             context = new SpringApplicationBuilder(BackendApplication.class)

src/main/java/com/petshop/backend/RuntimeClasspathValidator.java

@@ -0,0 +1,24 @@
+package com.petshop.backend;
+
+import java.net.URL;
+
+final class RuntimeClasspathValidator {
+
+    private RuntimeClasspathValidator() {
+    }
+
+    static void validate() {
+        if (!resourceExists("application.yml")) {
+            throw new IllegalStateException("Backend resources are missing from the runtime classpath. Reimport the Maven project in IntelliJ and run the shared Maven run configuration.");
+        }
+        if (!resourceExists("db/migration/V1__baseline_schema.sql")) {
+            throw new IllegalStateException("Flyway migration files are missing from the runtime classpath. Reimport the Maven project in IntelliJ and run the shared Maven run configuration.");
+        }
+    }
+
+    private static boolean resourceExists(String path) {
+        ClassLoader classLoader = RuntimeClasspathValidator.class.getClassLoader();
+        URL resource = classLoader.getResource(path);
+        return resource != null;
+    }
+}