feat: modernize application — source, platform artifacts, CI/CD (#3)

docs/architecture.md

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+# Architecture
+
+## Service Design
+
+demo-sam-1 is a microservice following cloud-native patterns.
+
+## Technology Stack
+
+- **Runtime**: Java Spring Boot
+- **Deployment**: Humanitec Platform Orchestrator
+- **CI/CD**: Gitea Actions → ACR → Humanitec
+
+## Dependencies
+
+See `score.yaml` for external resource dependencies.