feat: rewrite README to reflect actual app

[PlayerNguyen]

Summary

Replaces the generic boilerplate README with accurate documentation that reflects the current state of the application.

Changes

• Tech stack: Documents actual stack — Bun, Hono, React 19, Prisma/PostgreSQL, Redis, Mantine UI, TanStack Router, Zustand, Zod
• Project structure: Adds accurate monorepo layout showing apps/api, apps/ui, and packages/*
• Features: Lists real features — JWT auth (HS512), RBAC with granular permissions, Redis caching with TTL, OpenAPI/Swagger auto-generation, paginated endpoints
• Setup instructions: Corrects to use bun and docker compose (PostgreSQL, not MySQL)
• Scripts: Documents all available monorepo scripts (dev, build, test, migrate, seed, generate)
• Environment variables: Lists all required env vars for both API and UI
• Default credentials: Includes test admin/user accounts
• CI: Documents the GitHub Actions workflow

Removed

• Generic dashboard widget/layout code examples (not part of the actual app)
• Placeholder contribution guidelines and support links
• Inaccurate feature claims (custom widgets API, real-time streaming, theming system)