chore: scaffold plugin-bench MVP

[amh1k]

Summary

This PR adds the initial scaffolding for the OpenEverest Performance Benchmark plugin and documents the agreed MVP requirements in spec.md.

UI Preview

Changes

• Added the React frontend plugin entrypoint.
• Added the Go backend HTTP server.
• Added frontend bundle embedding through Go.
• Added health, status, and placeholder run-history endpoints.
• Added the Helm chart and OpenEverest Plugin resource.
• Added the Performance Benchmark tab to the database cluster overview.
• Restricted the tab to PostgreSQL providers:
  • provider-cloudnative-pg
  • provider-percona-postgresql
• Added Docker image packaging.
• Added CI validation for the frontend, backend, and Helm chart.
• Added Makefile commands for local validation.
• Added the MVP and next-stage requirements to spec.md.

Testing

• npm run build
• go vet ./...
• go test ./...
• go build -o /dev/null .
• helm lint charts/plugin-bench/
• git diff --check

Notes

The spec.md document will evolve as requirements, implementation details, and design decisions are refined during development.

Fixes #1