chore: upgrade Spring Boot 3.3.4 → 4.0.6

- Remove manually-pinned versions (Hibernate, Flyway, AssertJ, Mockito)
  now managed by Boot 4.0.6 BOM
- Remove @EntityScan and @EnableJpaRepositories — auto-detected via
  scanBasePackages covering de.cannamanage hierarchy
- All 25 tests pass, build compiles in 9.6s

pom.xml

@@ -7,7 +7,7 @@
     <parent>
         <groupId>org.springframework.boot</groupId>
         <artifactId>spring-boot-starter-parent</artifactId>
-        <version>3.3.4</version>
+        <version>4.0.6</version>
         <relativePath/>
     </parent>
 
@@ -30,11 +30,6 @@
         <maven.compiler.target>21</maven.compiler.target>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
-        <!-- Dependency versions -->
-        <hibernate.version>6.5.3.Final</hibernate.version>
-        <flyway.version>10.15.0</flyway.version>
-        <assertj.version>3.26.3</assertj.version>
-        <mockito.version>5.12.0</mockito.version>
         <!-- JaCoCo -->
         <jacoco.version>0.8.13</jacoco.version>
         <jacoco.minimum.coverage>1.00</jacoco.minimum.coverage>