refactor: migrate to YAML config and add Maven dev/prod profiles

- Replace application.properties with application.yaml (base/prod config)
  and application-dev.yaml (dev overrides: show-sql=true)
- Add Maven 'dev' profile (activeByDefault) and 'prod' profile to pom.xml;
  spring-boot:run picks up the active Spring profile automatically
- Guard DataInitializer.initData with @Profile("dev") so test data is
  never seeded in production

Local dev: ./mvnw spring-boot:run (dev profile active by default)
Production: SPRING_PROFILES_ACTIVE env var controls the Spring profile;
            Maven profiles are irrelevant for the packaged JAR.

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

backend/pom.xml

@@ -148,11 +148,34 @@
 	</dependencies>
 
 
+	<profiles>
+		<profile>
+			<id>dev</id>
+			<activation>
+				<activeByDefault>true</activeByDefault>
+			</activation>
+			<properties>
+				<spring.profiles.active>dev</spring.profiles.active>
+			</properties>
+		</profile>
+		<profile>
+			<id>prod</id>
+			<properties>
+				<spring.profiles.active></spring.profiles.active>
+			</properties>
+		</profile>
+	</profiles>
+
 	<build>
 		<plugins>
 			<plugin>
 				<groupId>org.springframework.boot</groupId>
 				<artifactId>spring-boot-maven-plugin</artifactId>
+				<configuration>
+					<profiles>
+						<profile>${spring.profiles.active}</profile>
+					</profiles>
+				</configuration>
 			</plugin>
 		</plugins>
 	</build>