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/setup.py

@@ -19,7 +19,7 @@ from app.utils.setup_state import is_setup_complete_cached, set_setup_complete_c
 
 log: structlog.stdlib.BoundLogger = structlog.get_logger()
 
-router = APIRouter(prefix="/api/setup", tags=["setup"])
+router = APIRouter(prefix="/api/v1/setup", tags=["setup"])
 
 
 @router.get(