Faster first library scan: bounded concurrency and a 32 KB first read

An uncached scan probed one image at a time and read 256 KB of each. On
a NAS that is bound by per-file latency (the server/Synology: ~20 ms per
image, ~100 min for 298k images). Now chapter folders of a book and
their images are probed SCAN_CONCURRENCY (default 8, 1-64) at a time
and assembled in folder order, and imageSize() reads 32 KB first, then
256 KB, then the whole file: a prefix gives either no size or the same
size, so results do not change. Tests: every format and read stage
equals the whole-file answer; concurrency 1 and 8 build the same index
and cache; the limiter keeps its bound and per-task errors. (BL-0043)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server.js

@@ -7,6 +7,7 @@
  *   SIGNUP=closed|open   (the very first account can always be created and becomes admin)
  *   SCAN_INTERVAL=10     minutes between shallow change checks, 0 = off
  *   FULL_SCAN_INTERVAL=1440  minutes between full reconciliations, 0 = off
+ *   SCAN_CONCURRENCY=8   chapter folders / images probed at once by an uncached scan (library.js)
  *   COOKIE_SECURE=1      mark the session cookie Secure (when served over HTTPS)
  *   SCRAPER_URL=http://127.0.0.1:8091   scraper daemon API
  *   SCRAPER_TOKEN=...                    shared daemon token (optional on loopback)