Read-ahead per reader; compose passes READ_AHEAD and PERF_TRACE; report fixes

Each user gets an own read-ahead queue, served in turn, so two people reading
at once no longer cancel each other's chapter. The report rounds phase times
and measures the quiet time before a slow storage probe correctly.

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

docker-compose.yml

@@ -61,6 +61,8 @@ services:
       DISCOVERY_HARVEST_AT: ${DISCOVERY_HARVEST_AT:-03:30}
       DISCOVERY_ANILIST_URL: ${DISCOVERY_ANILIST_URL:-}
       GREATEST_COVER_WARM: ${GREATEST_COVER_WARM:-1}
+      READ_AHEAD: ${READ_AHEAD:-1}
+      PERF_TRACE: ${PERF_TRACE:-0}
     networks: [backend, egress]
     healthcheck:
       test: [CMD, node, tools/healthcheck.js, reader]