Show a database target's archive file, size and last write in the target list (closes #397)

Each database target's row on the webhook page names its archive
file, as delivery.ArchivePath finds it, and shows its size on disk and
when it was last written, or "not created yet" when there is no file.
The figures come from the metadata of the file and its -wal only: the
size is the two together, and the last write the later of their
modification times, since a write lands in the -wal first.

Model: opus-5-5

internal/delivery/target_database_archive.go

@@ -515,6 +515,47 @@ func (w *archiveWriter) prune(expiry time.Duration) {
 	}
 }
 
+// ArchiveFileInfo is what the metadata of a database target's archive
+// file says about it.
+type ArchiveFileInfo struct {
+	// Size is the bytes on disk of the file and its -wal together.
+	Size int64
+
+	// Written is when the file or its -wal was last modified, whichever
+	// is later: a write lands in the -wal first.
+	Written time.Time
+}
+
+// StatArchive reads the metadata of the archive file at path and of
+// its -wal, without opening the archive. With no file at path, which is
+// so before the first write and after the operator moved it away, the
+// error wraps fs.ErrNotExist.
+func StatArchive(path string) (ArchiveFileInfo, error) {
+	file, err := os.Stat(path)
+	if err != nil {
+		return ArchiveFileInfo{}, err
+	}
+
+	info := ArchiveFileInfo{Size: file.Size(), Written: file.ModTime()}
+
+	wal, err := os.Stat(path + "-wal")
+	if errors.Is(err, fs.ErrNotExist) {
+		return info, nil
+	}
+
+	if err != nil {
+		return ArchiveFileInfo{}, err
+	}
+
+	info.Size += wal.Size()
+
+	if wal.ModTime().After(info.Written) {
+		info.Written = wal.ModTime()
+	}
+
+	return info, nil
+}
+
 // fileExists reports whether a path currently exists.
 func fileExists(path string) bool {
 	_, err := os.Stat(path)