By-model panel gains a segmented 1天/7天/全部 control. Selecting a
range fetches /api/stats?since=<RFC3339> and re-renders only that table
client-side, replicating the human/rate template helpers in JS; the
existing /api/stats since param already drives the server-side filter.
- internal/server/templates/admin.html: filter UI + JS, empty/loading states
- internal/server/server_test.go: render test for the filter controls
- scripts/test-instance.sh: build→start→stop→clean test-instance helper
