[FEATURE] Add `GET /api/creators/:address/campaigns` endpoint

**Category:** backend
**Project area:** backend/src/

**Implementation:**
- Returns all campaigns where `creator` matches address.
- Same filters and pagination as `GET /api/campaigns`.
- Validate Stellar address format.

**Acceptance Criteria:**
- Invalid address returns 400.
- Unknown address returns empty array.
- Consistent response schema with campaign list endpoint.