feat: implement API versioning /api/v1/

- All backend routers moved to /api/v1/ prefix
- Frontend BASE_URL updated to /api/v1
- Setup redirect middleware updated to redirect to /api/v1/setup
- Health router path fixed: prefix=/api/v1/health, @router.get('')
- conftest.py: set server_status=online for test fixture
- Created Docs/API_VERSIONING.md with deprecation policy
- Updated Docs/Backend-Development.md with versioning section
- Updated Instructions.md curl examples

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Docs/Instructions.md

@@ -230,11 +230,11 @@ The session cookie is named `bangui_session`.
 ```bash
 # Dev master password: Hallo123!
 HASHED=$(echo -n "Hallo123!" | sha256sum | awk '{print $1}')
-TOKEN=$(curl -s -X POST http://127.0.0.1:8000/api/auth/login \
+TOKEN=$(curl -s -X POST http://127.0.0.1:8000/api/v1/auth/login \
   -H 'Content-Type: application/json' \
   -d "{\"password\":\"$HASHED\"}" \
   | python3 -c 'import sys,json; print(json.load(sys.stdin)["token"])')
 
 # Use token in subsequent requests:
-curl -H "Cookie: bangui_session=$TOKEN" http://127.0.0.1:8000/api/dashboard/status
+curl -H "Cookie: bangui_session=$TOKEN" http://127.0.0.1:8000/api/v1/dashboard/status
 ```