Initial project setup: CI, Docker & configs

Add project bootstrap for build, CI/CD, and local development. Introduces Maven Wrapper (mvnw, mvnw.cmd, .mvn/wrapper), multi-stage Dockerfile, docker-compose files (compose.yaml, docker-compose.dev.yml), .env, banner and README. Adds Gitea Actions workflow to build, test and push container images. Update application.yml (port -> 8090, virtual threads, API versioning, management/actuator/prometheus settings, Steam API config) and add application-dev.yml for local overrides. Extend pom.xml with caching, observability (actuator, micrometer/prometheus), testing/dev dependencies (testcontainers, devtools, additional spring test modules) to support CI and local testing.

.mvn/wrapper/maven-wrapper.properties

@@ -0,0 +1,3 @@
+wrapperVersion=3.3.4
+distributionType=only-script
+distributionUrl=https://repo.maven.apache.org/maven2/org/apache/maven/apache-maven/3.9.14/apache-maven-3.9.14-bin.zip