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>

backend/app/routers/health.py

@@ -11,10 +11,10 @@ from fastapi.responses import JSONResponse
 
 from app.dependencies import ServerStatusDep
 
-router: APIRouter = APIRouter(prefix="/api", tags=["Health"])
+router: APIRouter = APIRouter(prefix="/api/v1/health", tags=["Health"])
 
 
-@router.get("/health", summary="Application health check")
+@router.get("", summary="Application health check")
 async def health_check(server_status: ServerStatusDep) -> JSONResponse:
     """Return application and fail2ban status.