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/routes/demo/+page.svelte

@@ -0,0 +1,5 @@
+<script lang="ts">
+	import { resolve } from '$app/paths';
+</script>
+
+<a href={resolve('/demo/paraglide')}>paraglide</a>