fix(backgrounds): stream unsplash download to temp file instead of memory

Use a temp file instead of io.ReadAll to avoid buffering the entire
Unsplash image in RAM, which could cause OOM with large images or
high maxsize configuration.

pkg/files/s3_test.go

@@ -367,7 +367,7 @@ func TestFileSave_S3_ReturnsErrorOnPutObjectFailure(t *testing.T) {
 	assert.Contains(t, err.Error(), "failed to upload file to S3")
 }
 
-func TestFileSave_S3_LogsWarnOnSizeMismatch(t *testing.T) {
+func TestFileSave_S3_UsesActualReaderSizeOnMismatch(t *testing.T) {
 	originalClient := s3Client
 	originalBucket := s3Bucket
 	t.Cleanup(func() {

pkg/modules/background/unsplash/unsplash.go

@@ -20,9 +20,11 @@ import (
 	"bytes"
 	"context"
 	"encoding/json"
+	"fmt"
 	"io"
 	"net/http"
 	"net/url"
+	"os"
 	"strconv"
 	"strings"
 	"time"
@@ -286,19 +288,31 @@ func (p *Provider) Set(s *xorm.Session, image *background.Image, project *models
 		return files.ErrFileIsTooLarge{Size: uint64(resp.ContentLength)}
 	}
 
-	// Buffer the response body so we have a seekable reader for S3 uploads.
+	// Stream the response body into a temp file so we have a seekable reader
+	// for S3 uploads without buffering the entire image in memory.
+	tmpFile, err := os.CreateTemp("", "vikunja-unsplash-*")
+	if err != nil {
+		return fmt.Errorf("could not create temp file for unsplash download: %w", err)
+	}
+	defer os.Remove(tmpFile.Name())
+	defer tmpFile.Close()
+
 	// Use LimitReader as a safety net in case Content-Length was missing or inaccurate.
 	limitedReader := io.LimitReader(resp.Body, int64(maxSize)+1) // #nosec G115 -- maxSize is configured, not user input
-	bodyBytes, err := io.ReadAll(limitedReader)
+	written, err := io.Copy(tmpFile, limitedReader)
 	if err != nil {
-		return err
+		return fmt.Errorf("could not write unsplash download to temp file: %w", err)
 	}
-	if uint64(len(bodyBytes)) > maxSize {
-		return files.ErrFileIsTooLarge{Size: uint64(len(bodyBytes))}
+	if uint64(written) > maxSize {
+		return files.ErrFileIsTooLarge{Size: uint64(written)}
+	}
+
+	if _, err = tmpFile.Seek(0, io.SeekStart); err != nil {
+		return fmt.Errorf("could not seek temp file to start: %w", err)
 	}
 
 	// Save it as a file in vikunja
-	file, err := files.Create(bytes.NewReader(bodyBytes), "", uint64(len(bodyBytes)), auth)
+	file, err := files.Create(tmpFile, "", uint64(written), auth)
 	if err != nil {
 		return
 	}