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>

docker-compose.yml

@@ -50,6 +50,7 @@ services:
       TZ: ${TZ:-Europe/Berlin}
       SIGNUP: closed
       SCAN_INTERVAL: "10"
+      SCAN_CONCURRENCY: ${SCAN_CONCURRENCY:-8}
       COOKIE_SECURE: ${COOKIE_SECURE:-0}
       READING_WINDOWS_JSON: ${READING_WINDOWS_JSON:-}
       READING_TIMEZONE: ${READING_TIMEZONE:-Europe/Berlin}