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.yml

@@ -0,0 +1,23 @@
+spring:
+  application:
+    name: cameleer-saas
+  jpa:
+    open-in-view: false
+    hibernate:
+      ddl-auto: validate
+  flyway:
+    enabled: true
+    locations: classpath:db/migration
+
+management:
+  endpoints:
+    web:
+      exposure:
+        include: health,info
+  endpoint:
+    health:
+      show-details: when-authorized
+
+cameleer:
+  jwt:
+    expiration: 86400 # 24 hours in seconds