feat(banners): instance announcements — admin CRUD, top bar, CLI (#39)

banners table (title/body/level/active/dismissible/schedule window).
GET /banners/active for signed-in users; admin CRUD under /admin/banners.
BannerBar renders above the header with per-banner dismissal
(localStorage); admin console gains an Announcements section;
`containr banners` manages them and `auth status` prints active ones.

Generated with [Devin](https://devin.ai)

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/backend/internal/cli/root.go

@@ -89,6 +89,7 @@ func init() {
 	rootCmd.AddCommand(commands.NotificationsCmd)
 	rootCmd.AddCommand(commands.WebhooksCmd)
 	rootCmd.AddCommand(commands.OperationsCmd)
+	rootCmd.AddCommand(commands.BannersCmd)
 	rootCmd.AddCommand(commands.UpCmd)
 }