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>

.gitignore

@@ -0,0 +1,20 @@
+# Maven
+target/
+
+# IDE
+.idea/
+*.iml
+.vscode/
+.settings/
+.classpath
+.project
+.factorypath
+*.swp
+
+# OS
+.DS_Store
+Thumbs.db
+
+# Environment
+.env
+*.env.local