feat(dashboard): real listeners-over-time chart sourced from listener_histories

The dashboard had a placeholder bar chart that always rendered fake
data. Replace it with a per-mount SVG line chart that pulls actual
samples from the listener_histories table via /admin/insights.

Backend
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handleInsights now accepts ?hours=N (default 24, max 168 = 7d) and
?max_points=N (default 200, max 2000). Series longer than max_points
are bucket-averaged server-side so a 7-day view does not ship ~10k
rows per stream.

Frontend
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New <ListenerHistoryChart range={1H|24H|7D}/>: hand-rolled SVG line
chart, no third-party dep, ~240 LOC. One coloured polyline per source
mount with a legend showing peak listeners; auto-MP3 transcoder
mirrors are filtered from the legend (their curve is identical to
their source by construction). Y-axis auto-scales to peak; X-axis
labels adapt to the chosen window.

Time-range tabs that were already present on the dashboard now
actually drive the fetch instead of being decorative. Cached per
range so flipping tabs does not refetch.

Author: DatanoiseTV