docs: rewrite README with comprehensive documentation

[kiro-agent]

This pull request was generated by @kiro-agent 👻

Comment with /kiro fix to address specific feedback or /kiro all to address everything.
_{Learn about Kiro autonomous agent}

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Summary

Rewrites the README.md from a minimal set of instructions into a comprehensive project document.

What changed

• Architecture section with ASCII diagram and step-by-step request flow explanation
• AWS Services table listing all six services used and their purpose
• Annotated project structure explaining every significant file
• Prerequisites table with specific version requirements (Java 17, Maven 3.8+, CDK 2.x, AWS CLI, jq, curl)
• Detailed build, deploy, and test instructions including first-time CDK bootstrap and manual curl examples
• API reference with request/response JSON examples
• Data model documenting Unicorn fields and EventBridge event types
• Load testing instructions using the included Artillery config
• Cleanup instructions to tear down all AWS resources
• Key implementation details covering SnapStart, async init, CRT HTTP client, Tomcat exclusion, and Spring Boot 3 Jakarta EE migration
• References linking to all relevant AWS docs, libraries, and the workshop
• Preserved existing screenshot references (img/result.png, img/logs.png)

What was tested

• Verified all source file references (class names, package paths, dependency versions) match the actual codebase
• Confirmed all external links are valid