Add built-in backup of the data folder (BL-0044)

The reader owns a scheduled backup into BACKUP (Compose: BACKUP_PATH mounted at
/backup, off when unset). A run pauses the scraper queue through its control
API, waits (bounded) until no job is active, writes a verified tar.gz of DATA
with db.json from memory and scraper/queue.json stored paused, excludes
runtime/lock/pid/temp files, resumes only a queue it paused, then applies
count- or day-based retention to its own archive names only. Schedule,
retention, "Back up now" and last result live in Settings > Service health.

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

docker-compose.yml

@@ -36,10 +36,17 @@ services:
         source: ${DATA_PATH:-./data}
         target: /data
         bind: {create_host_path: false}
+      # Built-in backups (docs/backup.md). The folder must exist and be writable by
+      # uid 1000; backups stay off unless BACKUP_PATH is set.
+      - type: bind
+        source: ${BACKUP_PATH:-./backup}
+        target: /backup
+        bind: {create_host_path: false}
     environment:
       PORT: "8080"
       DATA: /data
       LIBRARY: /library
+      BACKUP: ${BACKUP_PATH:+/backup}
       TZ: ${TZ:-Europe/Berlin}
       SIGNUP: closed
       SCAN_INTERVAL: "10"