feat(evidence): add upload types and API client for frontend

- FileUploadResponse and PendingUpload types in types/upload.ts
- uploadsApi with upload(), getUrl(), list(), remove() methods
- Exported from types/index.ts and api/index.ts

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/api/index.ts

@@ -29,3 +29,4 @@ export { flowProposalsApi } from './flowProposals'
 export { flowpilotAnalyticsApi } from './flowpilotAnalytics'
 export { notificationsApi } from './notifications'
 export { publicTemplatesApi } from './publicTemplates'
+export { uploadsApi, default as uploadsApiDefault } from './uploads'