Add API documentation with example requests

[wisdomqin]

Description

The project has 18 API modules but no standalone API documentation. Adding a docs/api.md with endpoint descriptions and example requests would help both contributors and users.

What to do

1. Create docs/api.md
2. Document key API groups with endpoints, methods, and example curl commands:
   • Auth (/api/auth/)
   • Agents (/api/agents/)
   • Tasks (/api/tasks/)
   • Files (/api/files/)
3. FastAPI auto-generates docs at /docs — use that as reference

Resources

• API routes: backend/app/api/ (18 router files)
• FastAPI Swagger UI: http://localhost:8008/docs (when running)