docs: finalize Happy Post deployment readiness evidence

[afaryy]

Summary

• record successful P6 database-backed deployment and backend restart persistence evidence
• document CloudWatch log groups, ECS Container Insights, and operational log locations
• embed the high-level solution and Terraform ownership Mermaid diagrams in architecture documentation
• clean up the implementation backlog so remaining work is future hardening
• document why Terraform roots are explicit and why a modules directory is deferred until reuse appears
• add prominent README links for architecture, the live sandbox application, and public smoke-test endpoints

Validation

• Markdown local link check
• SVG XML check
• Mermaid source presence check
• git diff --check
• final documentation-only scope check

Notes

• documentation-only PR
• no application, Terraform resource behavior, workflow, CloudFormation, AWS resource, Dockerfile, diagram artifact, or image changes

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud