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/migrations_goose/20261104000000_banners.sql

@@ -0,0 +1,21 @@
+-- +goose Up
+-- Instance-wide announcement banners. Admins create them; active ones are
+-- shown to every signed-in user in the web UI and surfaced in the CLI.
+CREATE TABLE IF NOT EXISTS 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 timestamptz,
+    ends_at timestamptz,
+    created_at timestamptz DEFAULT now(),
+    updated_at timestamptz DEFAULT now()
+);
+
+CREATE INDEX IF NOT EXISTS idx_banners_active ON banners(active);
+
+-- +goose Down
+DROP TABLE IF EXISTS banners;