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'

frontend/src/api/uploads.ts

@@ -0,0 +1,32 @@
+import apiClient from './client'
+import type { FileUploadResponse } from '@/types/upload'
+
+export const uploadsApi = {
+  async upload(file: File, sessionId?: string): Promise<FileUploadResponse> {
+    const formData = new FormData()
+    formData.append('file', file)
+    if (sessionId) formData.append('session_id', sessionId)
+    const response = await apiClient.post<FileUploadResponse>('/uploads', formData, {
+      headers: { 'Content-Type': 'multipart/form-data' },
+    })
+    return response.data
+  },
+
+  async getUrl(id: string): Promise<string> {
+    const response = await apiClient.get<{ url: string }>(`/uploads/${id}/url`)
+    return response.data.url
+  },
+
+  async list(sessionId: string): Promise<FileUploadResponse[]> {
+    const response = await apiClient.get<FileUploadResponse[]>('/uploads', {
+      params: { session_id: sessionId },
+    })
+    return response.data
+  },
+
+  async remove(id: string): Promise<void> {
+    await apiClient.delete(`/uploads/${id}`)
+  },
+}
+
+export default uploadsApi