fix(analysis): cap HTTP backfill on CPU-seed pipeline load (#873)

* fix(analysis): cap HTTP backfill on CPU-seed pipeline load

Aggressive library analytics with multiple workers grew RAM unbounded:
HTTP downloads finish in ~500 ms, but Symphonia decode + R128 loudness
take seconds, so decoded `Vec<u8>` track buffers piled up in the
CPU-seed queue while the HTTP worker kept fetching. On large libraries
the process eventually saturated memory and forced the system into swap.

Backpressure: the HTTP backfill worker now checks the CPU-seed pipeline
depth (queued + running) against a `workers * 2` cap before popping the
next job. High-priority (now-playing) jobs bypass the cap so playback
prefetch is never starved. The CPU-seed worker pings the HTTP queue
after every completed decode, so the gate releases the instant decode
catches up.

The frontend backfill loop already idles at its own watermark when the
HTTP queue is satisfied — this change keeps the pipeline self-limiting
end-to-end even when callers (library top-up, playlist enqueue) submit
faster than decode drains.

Tests cover cap scaling, the floor for workers=1, the idle decision,
and the high-priority bypass.

* docs(changelog): analytics aggressive scan no longer eats memory (#873)

CHANGELOG.md

@@ -365,6 +365,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 
 
+### Analytics — aggressive scan no longer eats memory on big libraries
+
+**By [@Psychotoxical](https://github.com/Psychotoxical), PR [#873](https://github.com/Psychotoxical/psysonic/pull/873)**
+
+* **Settings → Library → Analytics → Aggressive** on multi-server or 100k+ track libraries no longer climbs in memory until the system swaps (Linux) or runs out (Windows OOM mid-scan): the HTTP download stage now waits for Symphonia decode + loudness to catch up instead of buffering tracks faster than they can be processed.
+* Now-playing prefetch still bypasses the cap, so starting a track during a background scan stays instant.
+
+
+
 ## [1.46.0] - 2026-05-18
 
 > **🙏 Special thanks to [@zz5zz](https://github.com/zz5zz)** for his tireless quirk-spotting and bug reports on the [Psysonic Discord](https://discord.gg/AMnDRErm4u) — several of the polish fixes in this release landed directly off the back of his messages.