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.

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

docs/guides/CLI.md

@@ -84,6 +84,8 @@ webhooks list|create|update|delete|deliveries|test
                                deliveries are HMAC-signed (X-Containr-Signature)
 operations                       live ops view: active deploys, queue depth,
                                24h failures, cron runs, backups
+banners list|create|update|delete  announcement banners (admin);
+                               active banners print on `auth status`
 up                               deploy the current directory
 ```