fix(charts): serve the All range's coarse buckets as their average day

The All range is read from the WEEK or MONTH rollup tier once a history is
long enough, from DAY rows for a short one, and from the live table on a
coverage miss. The chart folds every path into the average day of each
week or month, but a tier row carried a cumulative metric's whole total
and a level's reading-weighted mean. A month of steps therefore read about
thirty times higher for a long history than for a short one, under a
caption that said average.

A tier row now carries the average day of its bucket on every tier: a
cumulative metric's total over the days that hold a reading, a level's
mean of its daily means. The rollup fold counts the bucket's days and adds
up their means for it. Only the web chart's series reads (the chart
endpoint and the dashboard batch) use these rows.

docs/api/openapi.yaml

@@ -40744,7 +40744,9 @@ components:
                   $ref: "#/components/schemas/MeasurementType"
                 value:
                   type: number
-                  description: Daily mean (spot metrics) or SUM (cumulative).
+                  description: "The day's mean (spot metrics) or total (cumulative). A row of a coarser tier (a window longer than the
+                    daily cap) carries the average day of its bucket, the same quantity: a spot metric's mean of its
+                    daily means, a cumulative metric's total over the days of the bucket that hold a reading."
                 measuredAt:
                   type: string
                   format: date-time