restructure: flatten workspace nesting, move devcontainer to root

- backend/workspaces/backend/ → backend/
- backend/workspaces/frontend/ → frontend/
- backend/.devcontainer/ + .vscode/ → repo root (where VS Code expects them)
- loose scripts/SQL files → scripts/
- replace nested git repo with single repo at project root
- update docker-compose.yml build context and devcontainer.json path
- add root .gitignore

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/demo.spec.ts

@@ -0,0 +1,7 @@
+import { describe, it, expect } from 'vitest';
+
+describe('sum test', () => {
+	it('adds 1 + 2 to equal 3', () => {
+		expect(1 + 2).toBe(3);
+	});
+});