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>

backend/Dockerfile

@@ -0,0 +1,8 @@
+# Wir nutzen Java 21 (LTS), da Spring Boot 3 das empfiehlt
+FROM mcr.microsoft.com/devcontainers/java:1-21-bullseye
+
+# Optional: Zusätzliche OS-Pakete installieren
+# RUN apt-get update && apt-get install -y <package-name>
+
+# Port für Spring Boot
+EXPOSE 8080