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>

app/backend/sqlc/schema.sql

@@ -302,3 +302,17 @@ CREATE TABLE webhook_deliveries (
     created_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
     delivered_at TIMESTAMP WITH TIME ZONE
 );
+
+CREATE TABLE banners (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    title VARCHAR(255) NOT NULL,
+    body TEXT NOT NULL DEFAULT '',
+    level VARCHAR(20) NOT NULL DEFAULT 'info',
+    active BOOLEAN NOT NULL DEFAULT true,
+    dismissible BOOLEAN NOT NULL DEFAULT true,
+    created_by UUID REFERENCES users(id) ON DELETE SET NULL,
+    starts_at TIMESTAMP WITH TIME ZONE,
+    ends_at TIMESTAMP WITH TIME ZONE,
+    created_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
+    updated_at TIMESTAMP WITH TIME ZONE DEFAULT NOW()
+);