Auth Contract Baseline: Typed Entrypoints + Rotation-Ready Access Token Signer

[yasinmuhd]

Summary

Minimal auth milestone slice for assigned open issues in this account.

Included

• Added rotation-ready access-token signing abstraction:
  • apps/api/src/auth/token-signer.ts
  • AccessTokenSigner interface + default StaticKeyAccessTokenSigner
• Wired signer into auth responses:
  • register and login now issue signer-generated session.accessToken
• Added contributor-facing API auth entrypoint and extension documentation:
  • docs/auth-api-entrypoints.md
  • defines entrypoints, type-contract rules via @lumen/types, signer extension rules, and verification notes

Verification Notes

• Register/login now return typed session payloads with generated access tokens through the signer abstraction.
• Error normalization and existing /auth/me behavior remain unchanged in this minimal slice.

Closes #468
Closes #470
Closes #471