fix(users): validate profile updates at service boundary

[blackmore-technology-group]

Summary

Fixes #534.

UserService.updateProfile now validates at the service boundary instead of assuming every caller passed through the HTTP/Zod layer. It reuses the existing updateProfileSchema, so direct/internal callers get the same field limits, enum/type checks, and text sanitization without duplicating validation constants.

Validation failures are translated to the project's existing ValidationError shape before any database update occurs.

Tests

• Added service-level regression coverage for an over-length displayName.
• Added service-level regression coverage for null supplied to a string field by a direct/internal caller.
• Both cases verify that db.update is never reached.
• Targeted Vitest: 2/2 passed.
• ESLint on the changed service and test: passed.

Repository baseline note

npm run typecheck is not currently a usable repo-wide gate on current main: it reports pre-existing syntax errors in unrelated files including src/config/index.ts, admin/credentials/quizzes/rewards services, and src/stellar/client.ts. This PR does not touch those files.

The scoped regression and lint gates for this change are green.