add per-account quota for total message size disk usage

so a single user cannot fill up the disk.
by default, there is (still) no limit. a default can be set in the config file
for all accounts, and a per-account max size can be set that would override any
global setting.

this does not take into account disk usage of the index database. and also not
of any file system overhead.

ctl.go

@@ -685,11 +685,13 @@ func servectlcmd(ctx context.Context, ctl *ctl, shutdown func()) {
 		acc.WithWLock(func() {
 			var changes []store.Change
 			err = acc.DB.Write(ctx, func(tx *bstore.Tx) error {
-				return bstore.QueryTx[store.Mailbox](tx).ForEach(func(mb store.Mailbox) error {
+				var totalSize int64
+				err := bstore.QueryTx[store.Mailbox](tx).ForEach(func(mb store.Mailbox) error {
 					mc, err := mb.CalculateCounts(tx)
 					if err != nil {
 						return fmt.Errorf("calculating counts for mailbox %q: %w", mb.Name, err)
 					}
+					totalSize += mc.Size
 
 					if !mb.HaveCounts || mc != mb.MailboxCounts {
 						_, err := fmt.Fprintf(w, "for %s setting new counts %s (was %s)\n", mb.Name, mc, mb.MailboxCounts)
@@ -703,6 +705,23 @@ func servectlcmd(ctx context.Context, ctl *ctl, shutdown func()) {
 					}
 					return nil
 				})
+				if err != nil {
+					return err
+				}
+
+				du := store.DiskUsage{ID: 1}
+				if err := tx.Get(&du); err != nil {
+					return fmt.Errorf("get disk usage: %v", err)
+				}
+				if du.MessageSize != totalSize {
+					_, err := fmt.Fprintf(w, "setting new total message size %d (was %d)\n", totalSize, du.MessageSize)
+					ctl.xcheck(err, "write")
+					du.MessageSize = totalSize
+					if err := tx.Update(&du); err != nil {
+						return fmt.Errorf("update disk usage: %v", err)
+					}
+				}
+				return nil
 			})
 			ctl.xcheck(err, "write transaction for mailbox counts")