fix(task-watcher): _shared를 빌드 컨텍스트에 포함 (heartbeat import 크래시 수정)

task-watcher는 build context가 ./task-watcher라 services/_shared가 이미지에
없었음 → A3가 추가한 `from _shared.heartbeat import` ModuleNotFoundError로
컨테이너 즉시 크래시(재시작 후 alive=false). render 워커 패턴대로
context=. + COPY _shared /app/_shared + PYTHONPATH=/app 로 수정.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019LV86jBozkNhSFXJA412fq

services/task-watcher/Dockerfile

@@ -7,10 +7,13 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     ca-certificates tzdata \
  && rm -rf /var/lib/apt/lists/*
 
-COPY requirements.txt .
+COPY task-watcher/requirements.txt /app/
 RUN pip install --no-cache-dir --timeout 600 --retries 5 -r requirements.txt
 
-COPY . .
+# 공통 heartbeat 모듈 (services/_shared) — watcher.py가 from _shared.heartbeat import
+COPY _shared /app/_shared
+COPY task-watcher/. /app/
+ENV PYTHONPATH=/app
 
 EXPOSE 8000
 CMD ["python", "-m", "uvicorn", "main:app", "--host", "0.0.0.0", "--port", "8000", "--workers", "1"]