Shared error-code taxonomy module

[kilodesodiq-arch]

Problem Statement. Error codes (e.g. INVALID_TOKEN, INTERNAL_SERVER_ERROR,
AI_SERVICE_ERROR, HTTP_500) are scattered: each handler constructs its own ad-hoc
envelope. error-handling.e2e-spec.ts shows the code field is number for backend
errors but string for AI service errors.

Why it matters. ADR-env consumers want a stable code mapping; an SDK round-trip
through AI service then backend fails without a normalized taxonomy.

Technical Context. src/common/filters/http-exception.filter.ts,
app/ai-service/schemas/errors.py.

Expected Outcome. A shared src/common/errors/codes.ts with ErrorCode = enum
membership, each code carrying human-readable description and HTTP status. Both repos
generate code from docs/errors.yaml.

Acceptance Criteria.

• New module emits codes that are identical between backend and AI service.
• Tests in both repos assert the same code for the same error class.

Files or modules likely to be affected.
new src/common/errors/codes.ts,
app/ai-service/schemas/errors.py.

Difficulty. Medium
Estimated effort. M

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Backlog item #41 from `docs/maintainer-issue-backlog.md.

[ifygreg01-best]

i will love work on this issue i deliver very timely, i have also worked on project like this

[precy41]

Hi, I am a fullStack developer with years of experience, I have worked on so many project and I can submit a PR in less than 24hrs. Can I work on this please

[grantfox-oss]

🦊 GrantFox — @precy41 has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #249)
2. Your PR will be reviewed by the ChainForgee maintainers

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