feat(uploads): add native resumable upload support

Implement a native chunked resumable upload API and frontend integration
to support reliable large file uploads.

Changes include:
- Added a 3-step resumable upload API flow (create session, upload chunks, complete session).
- Introduced configuration options for chunk size, retention hours, and toggling the feature.
- Updated the frontend to utilize resumable uploads with progress tracking.
- Configured temporary chunk storage under `data/tmp/uploads` with automatic cleanup.
- Documented the API flow and configuration in the README.

backend/libs/handlers/upload.go

@@ -228,11 +228,22 @@ func (a *App) checkUploadPolicy(r *http.Request, user services.User, loggedIn bo
 	if len(files) == 0 {
 		return 0, ""
 	}
+	sizes := make([]int64, 0, len(files))
+	for _, file := range files {
+		sizes = append(sizes, file.Size)
+	}
+	return a.checkUploadPolicyForSizes(r, user, loggedIn, settings, policy, sizes, totalBytes)
+}
+
+func (a *App) checkUploadPolicyForSizes(r *http.Request, user services.User, loggedIn bool, settings services.UploadPolicySettings, policy services.EffectiveUploadPolicy, fileSizes []int64, totalBytes int64) (int, string) {
+	if len(fileSizes) == 0 {
+		return 0, ""
+	}
 	now := time.Now().UTC()
 	if policy.MaxUploadMB > 0 {
 		maxBytes := services.MegabytesToBytes(policy.MaxUploadMB)
-		for _, file := range files {
-			if file.Size > maxBytes {
+		for _, fileSize := range fileSizes {
+			if fileSize > maxBytes {
 				return http.StatusRequestEntityTooLarge, "file exceeds upload size limit"
 			}
 		}