Document API Error Codes and Responses

**Is your feature request related to a problem? Please describe.**
Currently, API error responses are returned by different endpoints, but there is **no centralized documentation** describing the possible error codes, their meanings, and when clients can expect them.

Because error responses are undocumented:
- API consumers have to infer behavior from trial and error
- Frontend and SDK developers cannot reliably handle errors
- New contributors struggle to understand expected failure cases

This makes integrations harder and increases ambiguity for API users.

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**Describe the solution you'd like**
Create a dedicated markdown document that clearly documents all API error responses in a single place.

The document should include:
- A list of common error codes (e.g. `UNAUTHORIZED`, `RATE_LIMIT_EXCEEDED`, `NOT_FOUND`)
- HTTP status codes associated with each error
- A short description explaining when each error occurs
- Example JSON error responses where applicable

This documentation should reflect existing API behavior and not introduce new error handling logic.

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**Describe alternatives you've considered**
- Relying on scattered inline documentation or code comments: difficult for API consumers to discover.
- Documenting errors only inside OpenAPI specs: less readable for humans and harder to maintain for high-level understanding.
- Letting clients infer errors dynamically: unreliable and unfriendly for developers.

A centralized markdown document provides the best balance of clarity and maintainability.

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**Are you going to work on implementing this?**

- [x] Yes
- [ ] No

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**Additional context**
This issue focuses purely on **documentation and standardization of existing error responses** and does not change API behavior.

Having a single source of truth for error codes will also make future work such as SDK generation, frontend error handling, and API versioning easier and more consistent.