Update aggregate API to use binary megabytes and enhance documentation

- Changed API responses and internal calculations to use binary megabytes (MiB) instead of octets for traffic metrics.
- Updated relevant endpoints in AGGREGATE.md to reflect the new metric units.
- Modified handler and merge logic to accommodate the new data structure and ensure accurate traffic reporting.
- Enhanced tests to validate the changes in traffic calculations and summary data.
- Deprecated the use of total_octets in favor of total_megabytes for consistency across the API.

internal/aggregate/units.go

@@ -0,0 +1,8 @@
+package aggregate
+
+// mebibyte — двоичный мегабайт (1024² октетов), в JSON полях *megabytes.
+const mebibyte = 1024 * 1024
+
+func octetsToMegabytes(o uint64) float64 {
+	return float64(o) / float64(mebibyte)
+}