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/api_tests/User.http

@@ -0,0 +1,17 @@
+### Schritt 1: Gruppen IDs herausfinden (als Admin)
+GET http://localhost:8080/api/admin/groups
+Authorization: Basic admin admin123
+
+### Schritt 2: Einen neuen User "Onkel_Heinz" anlegen
+POST http://localhost:8080/api/admin/users
+Authorization: Basic admin admin123
+Content-Type: application/json
+
+{
+  "username": "Onkel_Heinz",
+  "email": "heinz@familie.de",
+  "initialPassword": "geheim",
+  "groupIds": [
+    "26f33481-b68a-410d-852b-a081ca9ae930"
+  ]
+}