feat(creators): add creator onboarding & profile module

[Mozez155]

Introduce a CreatorProfile entity linked 1:1 to User so fans can onboard
as creators with public profile metadata before appearing in discovery.

• Entity: userId, unique handle, displayName, bio, bannerUrl, category,
  isOnboarded, timestamps; unique indexes on userId and handle
• POST /creators/onboard: authenticated user becomes a creator and is
  upgraded to the creator role; 409 on duplicate handle or re-onboard
• GET /creators/:handle: public profile (CreatorResponseDto, never exposes
  email/password or internal ids)
• PATCH /creators/me: owner updates displayName/bio/bannerUrl/category;
  ownership enforced (403 on mismatch), handle is immutable
• Validate handle format ^[a-z0-9_]{3,30}$ and uniqueness
• TypeORM migration with FK to users (CASCADE) and unique handle index
• Public CreatorResponseDto vs owner CreatorPrivateDto
• Seed one demo creator profile (handle: creator_one)
• Unit tests for handle validation and duplicate-handle conflict (35 tests)
• docs/creators-api.md with curl examples; Swagger documents all routes

closes #20