feat(backend): implement graceful shutdown for container stop

Graceful shutdown ensures in-flight operations complete before process exits:
- Lifespan shutdown handler drains pending tasks with 25s timeout
- Scheduler stops accepting new jobs immediately
- HTTP session, external logging, scheduler lock, DB conn closed cleanly
- 25s Python timeout leaves 5s margin before Docker's 30s SIGKILL

Files changed:
- backend/app/main.py: enhanced _lifespan shutdown with task drain
- Docker/Dockerfile.backend: documented signal handling in header
- Docker/docker-compose.yml: added stop_grace_period: 30s
- Docker/compose.prod.yml: added stop_grace_period: 30s
- Docs/Deployment.md: new Graceful Shutdown section with sequence table
- Docs/TROUBLESHOOTING.md: new Graceful Shutdown Issues section

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Docker/docker-compose.yml

@@ -38,6 +38,7 @@ services:
       image: git.lpl-mind.de/lukas.pupkalipinski/bangui/backend:latest
       container_name: bangui-backend
       restart: unless-stopped
+      stop_grace_period: 30s
       depends_on:
         fail2ban:
           condition: service_started