perf(insights): route daily chart fetches through rollup buckets

The Insights trends row fires three parallel
GET /api/measurements?type=…&aggregate=daily&limit=5000 requests on
mount (BP_SYS / BP_DIA / WEIGHT). The legacy date_trunc path scanned
the measurements table each time — ~3 s per call on Marc's account.

Add a `source=rollup` opt-in on the listMeasurements schema. When set
alongside `aggregate=daily` the route reads from measurement_rollups
DAY buckets instead, swapping a full-table scan for a small indexed
read against the ~5 k-row rollup table. Cumulative HK metrics get
mean * count to reconstruct the daily SUM the chart expects.

The chart-data client opts in for every windowDays > 7 fetch so the
trends row + every sub-page chart benefit. When the rollup returns
zero buckets for the requested window the route falls through to the
live aggregate so brand-new accounts still render their first chart
correctly.

docs/api/openapi.yaml

@@ -2492,6 +2492,12 @@ paths:
               - weekly
               - monthly
             type: string
+        - in: query
+          name: source
+          schema:
+            enum:
+              - rollup
+            type: string
       responses:
         "200":
           content: