fix: second-pass review bugs in server and core

- ExportRunner: stop batch on first error (was continuing, overwriting
  error status with done)
- Export route: validate input_path against MEDIA_DIRS
- Export route: validate encoder, portrait_ratio, folder_suffix, name
- Export route: fix format check for WebP sequence
- Export route: add _ separator in folder_suffix (match GUI)
- Export route: use realpath consistently in delete endpoint
- Export route: drop runner ref on completion (prevent memory leak)
- ProcessedDB: use cursor-level row_factory (thread-safe)
- WebSocket: catch all exceptions in connect, cleanup in finally
- Dockerfile: use uvicorn[standard] for websockets support

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

@@ -3,6 +3,6 @@ RUN apt-get update && apt-get install -y ffmpeg && rm -rf /var/lib/apt/lists/*
 WORKDIR /app
 COPY core/ core/
 COPY server/ server/
-RUN pip install --no-cache-dir fastapi uvicorn
+RUN pip install --no-cache-dir fastapi uvicorn[standard]
 EXPOSE 8000
 CMD ["uvicorn", "server.app:app", "--host", "0.0.0.0", "--port", "8000"]