feat(perf): add on-demand (ui) throughput to cover pipeline cpm (#947)

* feat(perf): add on-demand (ui) throughput to cover pipeline cpm

Cover cpm previously measured only the native backfill (lib) via the
cover:library-progress done delta. Add a parallel UI series: every completed
on-demand Rust cover ensure (grid / now-playing) records a timestamp, surfaced
as a covers-per-minute rate. Both are exposed in the live cover diag, shown as
separate Backfill (lib) / On-demand (ui) cards in the Monitor tab (each pinnable
to the overlay) and as lib/ui rows in the Cover pipeline overlay block.

* docs(changelog): note cover on-demand (ui) throughput (PR #947)

Add CHANGELOG entry and credits line for the UI cover cpm metric.

* fix(perf): source on-demand (ui) cpm from backend produced-cover count

The JS ensure-queue counter never tracked: produced covers return hit:true
(only misses/errors are hit:false), and ensure-queue dedup/HMR made client
counting unreliable. Count on-demand covers natively in ensure_inner on the
produce path (non-bulk, past the cache-hit gate), expose a cumulative
uiEnsuredTotal in the pipeline stats, and derive the per-minute rate on the
frontend from polled deltas — mirroring the lib backfill series.

* perf(cover): measure cpm over trailing 5s instead of full minute

A 60s rolling average added too much inertia, flattening real bursts and
stalls in both the lib backfill and on-demand (ui) cover throughput. Compute
the rate from the trailing 5s of samples (still extrapolated to per-minute),
so the figure reacts promptly and decays to 0 within the window when idle.

src/api/coverCache.ts

@@ -43,6 +43,7 @@ export type CoverPipelineQueueStatsDto = {
   libraryBackfillHttpMax: number;
   libraryBackfillHttpActive: number;
   libraryBackfillPassRunning: boolean;
+  uiEnsuredTotal: number;
 };
 
 let coverAutoDownloadEnabled = true;