Allow mixed-case museum codes while preventing case-insensitive duplicates

[karilint]

Summary

• remove the frontend museum validator restriction that required all-uppercase codes
• add backend duplicate detection for museum codes using case-insensitive comparison
• return a structured 409 conflict response when a case-insensitive duplicate code is submitted
• add API test coverage for mixed-case/dashed code acceptance and case-only duplicate rejection

Details

Users can now create museum codes such as MNHN-Bol (still disallowing spaces). The backend blocks creating another code that differs only by letter case (e.g. MNHN-BOL after MNHN-Bol).

Validation

• pre-commit checks ran automatically during commit (lint + tsc) and passed.

---

Codex Task