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/docker-compose.yml

@@ -82,7 +82,8 @@ services:
 
   task-watcher:
     build:
-      context: ./task-watcher
+      context: .
+      dockerfile: task-watcher/Dockerfile
     container_name: task-watcher
     restart: unless-stopped
     ports: