fix(backend): explicit Flyway bean to bypass broken auto-configuration

Spring Boot 4.0 Flyway auto-configuration is not triggering in the CI
environment — confirmed by empty DB and no flyway_schema_history table.
Replace YAML-based auto-config with an explicit @Bean that creates and
runs Flyway directly on startup, independent of any auto-configuration
conditions. Disable the auto-config via spring.flyway.enabled=false to
prevent interference. Add @DependsOn("flyway") to DataInitializer to
enforce that CommandLineRunner beans are only registered after migrations.

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

backend/src/main/resources/application.yaml

@@ -9,11 +9,7 @@ spring:
     driver-class-name: org.postgresql.Driver
 
   flyway:
-    enabled: true
-    locations: classpath:db/migration
-    url: ${SPRING_DATASOURCE_URL}
-    user: ${SPRING_DATASOURCE_USERNAME}
-    password: ${SPRING_DATASOURCE_PASSWORD}
+    enabled: false  # Managed explicitly via FlywayConfig bean
 
   jpa:
     hibernate: