_calculate_metadata()'s inline calc_stats() computed the median as a
naive sorted(data)[len(data) // 2], which returns the upper-middle
value for even-length datasets instead of averaging the two middle
values. A full day is always an even interval count (96 quarter-hours
or 24 hours), so this silently overstated "median" and
"median_position" in nearly every price_stats block of the
get_chartdata response. The rest of the codebase already has a
correct calculate_median() in utils/average.py; this was an isolated
inline duplicate with the bug.
Impact: get_chartdata's price_stats.combined/dayN.median and
median_position now report the statistically correct median for
chart annotations and metadata-driven dashboards.
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