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/messages/es.json

@@ -0,0 +1,4 @@
+{
+	"$schema": "https://inlang.com/schema/inlang-message-format",
+	"hello_world": "Hello, {name} from es!"
+}