Admin: analytics date-range toggle (7d / 30d / 90d)

/api/admin/stats accepts ?days= (clamped to 7/30/90, default 30) → passed to
admin_stats, which already windows visitors, retention, funnel, sharing, daily
trend, and the top lists by that span. Frontend adds a Window picker on the
analytics tabs (Overview/Content/Audience); changing it refetches and the
windowed labels ("Visitors (Nd)", "Last N days", "Returning visitors (Nd)")
follow. Corpus totals + source health are unaffected (not time-windowed).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_admin.py

@@ -97,3 +97,12 @@ def test_source_health_includes_metrics(tmp_path, monkeypatch):
                 "acceptance_rate", "duplicate_rate", "review_reason", "next_due_at"):
         assert key in s, f"missing {key}"
     assert s["served"] == 1 and s["acceptance_rate"] == 100
+
+
+def test_admin_stats_days_param_clamped(tmp_path, monkeypatch):
+    app, api = _make(tmp_path, monkeypatch, admin_email="boss@x.com")
+    tc = _signin(app, api, "boss@x.com")
+    assert tc.get("/api/admin/stats?days=7").json()["days"] == 7
+    assert tc.get("/api/admin/stats?days=90").json()["days"] == 90
+    assert tc.get("/api/admin/stats?days=999").json()["days"] == 30   # clamped
+    assert tc.get("/api/admin/stats").json()["days"] == 30            # default