Phase 4: backups w/ volumes, notifications, settings & users, audit page (0.4.0)

- Backup/restore: per-stack tar.gz incl. named-volume snapshots (helper
  container), upload restore with rename/overwrite/conflict detection.
- Notifications: ntfy/Discord/Slack/Gotify/generic webhooks, per-event
  subscriptions; wired into the update checker and stack lifecycle.
- Settings page: update-check interval, webhook CRUD + test, user management
  (with last-admin safeguards).
- Audit log page (searchable, paginated).
- Mobile-responsive sidebar/layout.

Multi-host agents and remote backup destinations (SFTP/S3) deferred.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/main.py

@@ -16,9 +16,11 @@ from docker_client import DockerError
 from routers import (
     audit,
     auth,
+    backups,
     editor,
     images,
     ports,
+    settings as settings_router,
     stacks,
     system,
     templates,
@@ -46,7 +48,7 @@ async def lifespan(app: FastAPI):
     update_task.cancel()
 
 
-app = FastAPI(title="StackPilot", version="0.3.0", lifespan=lifespan)
+app = FastAPI(title="StackPilot", version="0.4.0", lifespan=lifespan)
 
 app.add_middleware(
     CORSMiddleware,
@@ -74,6 +76,8 @@ app.include_router(images.router)
 app.include_router(ports.router)
 app.include_router(templates.router)
 app.include_router(audit.router)
+app.include_router(settings_router.router)
+app.include_router(backups.router)
 app.include_router(ws.router)