feat: summarize 전용 stage — 뉴스 AI 요약 (classify 없이)

- summarize_worker: 요약만 생성 (분류 안 함)
- queue_consumer: summarize stage 추가 (batch 3)
- news_collector: summarize + embed 큐 등록
- process_stage enum에 'summarize' 추가

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/models/queue.py

@@ -14,7 +14,7 @@ class ProcessingQueue(Base):
     id: Mapped[int] = mapped_column(BigInteger, primary_key=True)
     document_id: Mapped[int] = mapped_column(BigInteger, ForeignKey("documents.id"), nullable=False)
     stage: Mapped[str] = mapped_column(
-        Enum("extract", "classify", "embed", "preview", name="process_stage"), nullable=False
+        Enum("extract", "classify", "summarize", "embed", "preview", name="process_stage"), nullable=False
     )
     status: Mapped[str] = mapped_column(
         Enum("pending", "processing", "completed", "failed", name="process_status"),