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/Dockerfile.backend

@@ -7,6 +7,11 @@
 #  Usage:
 #    docker build -t bangui-backend -f Docker/Dockerfile.backend .
 #    podman build -t bangui-backend -f Docker/Dockerfile.backend .
+#
+#  Signal handling:
+#    - STOPSIGNAL defaults to SIGTERM (handled by uvicorn → lifespan shutdown)
+#    - stop_grace_period in docker-compose.yml controls Docker's kill timeout
+#    - Python code allows 25s for in-flight tasks to drain before hard kill
 # ──────────────────────────────────────────────────────────────
 
 # ── Stage 1: build dependencies ──────────────────────────────