feat(api): Add comprehensive REST API for Project Boards

Implements complete REST API for Gitea's Project Board feature with
10 new endpoints enabling programmatic project management.

Features:
- Project CRUD operations (create, read, update, delete, list)
- Column management (create, update, delete, list)
- Issue assignment and movement between columns
- State filtering (open/closed/all)
- Pagination support
- Comprehensive permission checks
- Full Swagger/OpenAPI documentation

API Endpoints:
- GET    /repos/{owner}/{repo}/projects
- POST   /repos/{owner}/{repo}/projects
- GET    /repos/{owner}/{repo}/projects/{id}
- PATCH  /repos/{owner}/{repo}/projects/{id}
- DELETE /repos/{owner}/{repo}/projects/{id}
- GET    /repos/{owner}/{repo}/projects/{id}/columns
- POST   /repos/{owner}/{repo}/projects/{id}/columns
- PATCH  /repos/{owner}/{repo}/projects/columns/{id}
- DELETE /repos/{owner}/{repo}/projects/columns/{id}
- POST   /repos/{owner}/{repo}/projects/columns/{id}/issues

Implementation follows Gitea's standard patterns:
- Router → Service → Model architecture
- Proper access control with token scopes
- Comprehensive error handling
- Full test coverage with 11 test functions

Files changed:
- routers/api/v1/repo/project.go (new, 710 lines)
- modules/structs/project.go (new, 139 lines)
- services/convert/project.go (new, 92 lines)
- models/project/issue.go (+61 lines for AddIssueToColumn)
- routers/api/v1/api.go (+17 lines for route registration)
- tests/integration/api_repo_project_test.go (new, 604 lines)
- docs/API_PROJECT_BOARD.md (new, comprehensive documentation)

No breaking changes. No migration required.

Author: SupenBysz