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/java/net/siegeln/cameleer/saas/CameleerSaasApplication.java

@@ -0,0 +1,11 @@
+package net.siegeln.cameleer.saas;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+
+@SpringBootApplication
+public class CameleerSaasApplication {
+    public static void main(String[] args) {
+        SpringApplication.run(CameleerSaasApplication.class, args);
+    }
+}