fix(analysis): bound concurrent OCR inferences with a shared semaphore

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/tests/analysis_ocr.rs

@@ -56,6 +56,7 @@ fn ocr_dispatcher(
         storage,
         engine: StubOcrEngine::new(lines),
         max_image_bytes: 8 * 1024 * 1024,
+        ocr_permits: Arc::new(tokio::sync::Semaphore::new(1)),
     }
 }