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>

Dockerfile

@@ -10,12 +10,12 @@ RUN CGO_ENABLED=0 go build -trimpath -ldflags "-s -w -X main.version=${VERSION}"
 FROM alpine:3
 RUN apk add --no-cache ca-certificates tzdata \
  && adduser -D -u 1000 canon \
- && mkdir -p /data && chown canon:canon /data
+ && mkdir -p /data /backups && chown canon:canon /data /backups
 COPY --from=build /out/canon /usr/local/bin/canon
 USER canon
 ENV CANON_DATA_DIR=/data \
     CANON_PORT=4242
-VOLUME /data
+VOLUME /data /backups
 EXPOSE 4242
 HEALTHCHECK --interval=30s --timeout=3s --start-period=5s \
   CMD wget -qO- http://127.0.0.1:${CANON_PORT}/healthz >/dev/null || exit 1