docs(readme): update quickstart, structure, endpoints, admin docs

[pierringshot]

Summary

• What change does this PR introduce and why?
• Link any related issue(s): Fixes #

Checklist

• Code follows project structure (route → controller → service → utils).
• Input validation added/updated where applicable (schemas/).
• No secrets added to code; config via .env only.
• Lint passes: npm run lint (from WhatsCore.AI_5.0.1/).
• Tests pass locally: SKIP_WHATSAPP=true npm test.
• Updated docs (AGENTS.md/README, Swagger docs/openapi.yaml if API changes).
• Considered backward compatibility; API contracts unchanged or documented.

Testing Notes

• Steps to reproduce and verify the change locally:
  1. cd WhatsCore.AI_5.0.1 && npm install
  2. SKIP_WHATSAPP=true npm test
  3. (If applicable) describe manual validation (e.g., /api calls)

Screenshots / Logs (optional)

Deployment Considerations

• Requires env/config changes
• Data migrations or one-off scripts