Add scheduled and manual backups

Backups are .tar.gz archives of a consistent database snapshot (VACUUM
INTO) plus the images folder. They run on a cron schedule, daily at 3:00
by default, and the newest N scheduled ones are kept (7 by default).
Manual backups are never pruned.

Settings → Backups offers schedule presets or a custom cron expression,
retention, "Back up now", and download/delete for each backup. A small
built-in parser handles standard 5-field cron syntax with names, ranges,
steps and @daily-style shorthands.

Backups go to CANON_BACKUP_DIR (default data/backups). Compose mounts a
separate canon-backups volume at /backups, overridable with
CANON_BACKUP_PATH.

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

docker-compose.yml

@@ -23,6 +23,11 @@ services:
       # A named volume by default, so a Git sync never touches the database.
       # Set CANON_DATA_PATH to a host folder (e.g. ./data) to keep it there.
       - ${CANON_DATA_PATH:-canon-data}:/data
+      # Backups get their own mount so they can live somewhere else, like a
+      # NAS share. Set CANON_BACKUP_PATH to a host folder to use one.
+      - ${CANON_BACKUP_PATH:-canon-backups}:/backups
+    environment:
+      CANON_BACKUP_DIR: /backups
     env_file:
       - path: .env
         required: false
@@ -31,6 +36,7 @@ services:
 
 volumes:
   canon-data:
+  canon-backups:
 
 networks:
   chromanet: