feat: initialize Maven project with Spring Boot 3.4.3 + Java 21

Set up the foundational Spring Boot project structure:
- Root POM with web, security, JPA, Flyway, validation, AOP, actuator
- PostgreSQL + Testcontainers for test infrastructure
- Application YAML configs for default, dev, and test profiles
- Maven wrapper (3.9.9) for reproducible builds
- .gitignore for Maven/IDE/OS artifacts

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/main/resources/application-test.yml

@@ -0,0 +1,5 @@
+spring:
+  jpa:
+    show-sql: false
+  flyway:
+    clean-disabled: false