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>

internal/backup/backup_test.go

@@ -0,0 +1,89 @@
+package backup
+
+import (
+	"archive/tar"
+	"compress/gzip"
+	"context"
+	"io"
+	"log/slog"
+	"os"
+	"path/filepath"
+	"slices"
+	"testing"
+
+	"github.com/cdnutter/canon/internal/store"
+)
+
+func TestCreateListPrune(t *testing.T) {
+	data := t.TempDir()
+	st, err := store.Open(data)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer st.Close()
+	images := filepath.Join(data, "images")
+	os.MkdirAll(images, 0o755)
+	os.WriteFile(filepath.Join(images, "abc.jpg"), []byte("jpeg"), 0o644)
+	os.WriteFile(filepath.Join(images, ".upload-123"), []byte("partial"), 0o644)
+
+	m := New(filepath.Join(data, "backups"), images, st, slog.New(slog.DiscardHandler))
+	ctx := context.Background()
+	manual, err := m.Create(ctx, true)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := archiveNames(t, filepath.Join(m.Dir(), manual.Name)); !slices.Equal(got, []string{"canon.db", "images/abc.jpg"}) {
+		t.Fatalf("archive holds %v", got)
+	}
+	for range 3 {
+		if _, err := m.Create(ctx, false); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if err := m.prune(2); err != nil {
+		t.Fatal(err)
+	}
+	list, _ := m.List()
+	var scheduled, manuals int
+	for _, b := range list {
+		if b.Manual {
+			manuals++
+		} else {
+			scheduled++
+		}
+	}
+	if scheduled != 2 || manuals != 1 {
+		t.Fatalf("after prune: %d scheduled, %d manual", scheduled, manuals)
+	}
+	if _, err := m.Path("../canon.db"); err == nil {
+		t.Fatal("path traversal accepted")
+	}
+	if err := m.Delete(manual.Name); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func archiveNames(t *testing.T, path string) []string {
+	t.Helper()
+	f, err := os.Open(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+	gz, err := gzip.NewReader(f)
+	if err != nil {
+		t.Fatal(err)
+	}
+	tr := tar.NewReader(gz)
+	var names []string
+	for {
+		h, err := tr.Next()
+		if err == io.EOF {
+			return names
+		}
+		if err != nil {
+			t.Fatal(err)
+		}
+		names = append(names, h.Name)
+	}
+}