fix: update PGDATA path and add merge-upstream skill

- Fix PGDATA to /var/lib/postgresql/data/18/docker for postgres:18-alpine
- Add merge-upstream skill for automated upstream sync workflow

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

deploy/docker-compose.yml

@@ -150,7 +150,7 @@ services:
       # postgres:18-alpine 默认 PGDATA=/var/lib/postgresql/18/docker（位于镜像声明的匿名卷 /var/lib/postgresql 内）。
       # 若不显式设置 PGDATA，则即使挂载了 postgres_data 到 /var/lib/postgresql/data，数据也不会落盘到该命名卷，
       # docker compose down/up 后会触发 initdb 重新初始化，导致用户/密码等数据丢失。
-      - PGDATA=/var/lib/postgresql/data
+      - PGDATA=/var/lib/postgresql/data/18/docker
       - POSTGRES_USER=${POSTGRES_USER:-sub2api}
       - POSTGRES_PASSWORD=${POSTGRES_PASSWORD:?POSTGRES_PASSWORD is required}
       - POSTGRES_DB=${POSTGRES_DB:-sub2api}