fix(workers): presegment/csb 이벤트루프 blocking I/O to_thread 오프로드

- presegment_worker: fitz open/get_toc(동기 blocking, live 스테이지)를 to_thread 로 — 거대/손상
  PDF 파싱이 같은 루프의 1분 consumer + FastAPI 요청을 수백 ms~초 정지시키던 것 해소.
- csb_collector: 50MB PDF write_bytes + read_bytes(해시)를 to_thread 로 (R5 동형).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/workers/csb_collector.py

@@ -140,7 +140,8 @@ async def _download_pdf(url: str, dest: Path) -> int:
     if len(resp.content) > _MAX_PDF_BYTES:
         raise FeedError(f"PDF 크기 초과 ({len(resp.content)} bytes): {url}")
     dest.parent.mkdir(parents=True, exist_ok=True)
-    dest.write_bytes(resp.content)
+    # 최대 50MB PDF write 는 동기 blocking — 이벤트루프 점유 회피 to_thread (R5 동형).
+    await asyncio.to_thread(dest.write_bytes, resp.content)
     return len(resp.content)
 
 
@@ -190,9 +191,11 @@ async def _ingest_pdf(session, page_slug: str, pdf_url: str) -> bool:
 
     dest = Path(settings.nas_mount_path) / rel_path
     size = await _download_pdf(pdf_url, dest)
+    # 50MB PDF read + sha256 는 동기 blocking(I/O+CPU) — 이벤트루프 점유 회피 to_thread (R5 동형).
+    file_hash = await asyncio.to_thread(lambda: hashlib.sha256(dest.read_bytes()).hexdigest())
     doc = Document(
         file_path=rel_path,
-        file_hash=hashlib.sha256(dest.read_bytes()).hexdigest(),
+        file_hash=file_hash,
         file_format="pdf",
         file_size=size,
         file_type="immutable",

app/workers/presegment_worker.py

@@ -497,12 +497,18 @@ async def process(document_id: int, session: AsyncSession) -> None:
         logger.warning(f"[presegment] id={document_id} file not found ({raw}) → extract")
         return
 
+    import asyncio
+
     import fitz  # PyMuPDF — extract_worker/marker_worker 와 동일 의존
 
+    def _read_toc(path: str):
+        # fitz open/get_toc 는 동기 blocking — live 스테이지라 이벤트루프(같은 루프의 1분 consumer +
+        # FastAPI 요청) 점유 회피 위해 to_thread 오프로드(거대/손상 PDF 파싱 수백 ms~초).
+        with fitz.open(path) as pdf:
+            return pdf.page_count, (pdf.get_toc(simple=True) or [])
+
     try:
-        with fitz.open(str(source)) as pdf:
-            page_count = pdf.page_count
-            toc = pdf.get_toc(simple=True) or []
+        page_count, toc = await asyncio.to_thread(_read_toc, str(source))
     except Exception as exc:
         # PDF 손상 등 — 분할 불가. 단일문서로 통과(extract 가 PyMuPDF/OCR 로 재시도하며 가시화).
         logger.warning(