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/types/index.ts

@@ -15,6 +15,7 @@ export type { AssistantChat, AssistantChatMessage, ChatListItem, ChatMessageResp
 export * from './ai-session'
 export * from './flow-proposal'
 export * from './flowpilot-analytics'
+export * from './upload'
 
 // API response wrapper types
 export interface PaginatedResponse<T> {

frontend/src/types/upload.ts

@@ -0,0 +1,17 @@
+export interface FileUploadResponse {
+  id: string
+  filename: string
+  content_type: string
+  size_bytes: number
+  url: string
+  created_at: string
+}
+
+export interface PendingUpload {
+  id: string
+  file: File
+  preview: string
+  status: 'uploading' | 'done' | 'error'
+  result?: FileUploadResponse
+  error?: string
+}