chore: migrate to pnpm and add lightweight monorepo tooling

[Loosand]

Summary

Migrate the project to a lightweight pnpm workspace managed by Turborepo.

This keeps the existing frontend and backend directories in place while providing unified development, validation, testing, and build commands from the repository root.

Key changes:

• Add a root pnpm workspace for frontend and backend
• Add Turborepo task orchestration
• Add the @deeix/web and @deeix/api workspace packages
• Add unified root commands for development, checks, tests, and builds
• Replace Bun with pnpm 10.17.0
• Add a root pnpm-lock.yaml
• Update Docker builds and GitHub Actions to use pnpm
• Update development and deployment documentation
• Preserve the existing Go module and frontend directory structure

Change type

• Bug fix
• Feature
• Documentation
• Refactor
• Configuration / deployment
• Security hardening
• Other

Affected areas

• Frontend / UI
• Backend / API
• Authentication / authorization
• Conversations / streaming
• Files / RAG / extraction
• Model routing / providers
• MCP / tools
• Billing / payments
• Admin console
• Deployment / Docker / configuration
• Documentation

Verification

The following commands completed successfully:

pnpm install --frozen-lockfile
pnpm check
pnpm test
pnpm build