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/jails.py

@@ -44,7 +44,7 @@ from app.models.jail import (
 )
 from app.services import jail_service
 
-router: APIRouter = APIRouter(prefix="/api/jails", tags=["Jails"])
+router: APIRouter = APIRouter(prefix="/api/v1/jails", tags=["Jails"])
 
 _NamePath = Annotated[str, Path(description="Jail name as configured in fail2ban.")]