fix(backend): explicit Flyway config and DataInitializer null title fix

Adding explicit spring.flyway.* config (url/user/password) ensures Flyway
creates its own JDBC connection and runs migrations independently of the JPA
datasource initialization order in Spring Boot 4.0.

Fix DataInitializer creating a Document with title=null, which would hit the
NOT NULL constraint in the documents table once the admin user init succeeds.

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

backend/src/main/resources/application.yaml

@@ -8,6 +8,13 @@ spring:
     password: ${SPRING_DATASOURCE_PASSWORD}
     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}
+
   jpa:
     hibernate:
       ddl-auto: none