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/Document.http

@@ -0,0 +1,23 @@
+### 1. Upload Test (MIT AUTH)
+POST http://localhost:8080/api/documents/upload
+Authorization: Basic admin admin123
+Content-Type: multipart/form-data; boundary=boundary
+
+--boundary
+Content-Disposition: form-data; name="file"; filename="scan_79.pdf"
+Content-Type: application/pdf
+
+< ./demo.pdf
+--boundary--
+
+### 2. Excel Import Test (MIT AUTH)
+POST http://localhost:8080/api/documents/import-excel?overwrite=true
+Authorization: Basic admin admin123
+Content-Type: multipart/form-data; boundary=boundary2
+
+--boundary2
+Content-Disposition: form-data; name="file"; filename="metadata.xlsx"
+Content-Type: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
+
+< ./metadata.xlsx
+--boundary2--