chore(deploy): document ARCHIVE_RETENTION_DAYS in compose and .env.example

Days an archived item is kept with its reading history before library
scans purge it for good (default 90). Set 0 to keep archived items until
purged manually from the library admin page.

docker-compose.yml

@@ -61,6 +61,12 @@ services:
       BOOKHOARD_CONVERSION_TOOL: ${BOOKHOARD_CONVERSION_TOOL:-/usr/bin/kepubify}
       BOOKHOARD_CONVERSION_CACHE_TTL: ${BOOKHOARD_CONVERSION_CACHE_TTL:-24h}
 
+      # Library Maintenance
+      # Days an archived item (file missing from disk for 2+ scans) is kept,
+      # with its reading history, before library scans purge it for good.
+      # Set 0 to keep archived items until purged manually on the library admin page.
+      ARCHIVE_RETENTION_DAYS: ${ARCHIVE_RETENTION_DAYS:-90}
+
       # System timezone (fallback for server-side time operations)
       TZ: ${TZ:-UTC}
     ports: