feat: add notifications inbox db (#16599)

This PR is linked [to the following
issue](https://github.com/coder/internal/issues/334).

The objective is to create the DB layer and migration for the new `Coder
Inbox`.

coderd/database/migrations/testdata/fixtures/000297_notifications_inbox.up.sql

@@ -0,0 +1,25 @@
+INSERT INTO
+	inbox_notifications (
+		id,
+		user_id,
+		template_id,
+		targets,
+		title,
+		content,
+		icon,
+		actions,
+		read_at,
+		created_at
+	)
+	VALUES (
+		'68b396aa-7f53-4bf1-b8d8-4cbf5fa244e5', -- uuid
+		'5755e622-fadd-44ca-98da-5df070491844', -- uuid
+		'a0eebc99-9c0b-4ef8-bb6d-6bb9bd380a11', -- uuid
+        ARRAY[]::UUID[], -- uuid[]
+		'Test Notification',
+		'This is a test notification',
+		'https://test.coder.com/favicon.ico',
+		'{}',
+		'2025-01-01 00:00:00',
+		'2025-01-01 00:00:00'
+	);