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.

verifydata.go

@@ -265,6 +265,7 @@ possibly making them potentially no longer readable by the previous version.
 			checkf(err, dbpath, "reading mailboxes to check uidnext consistency")
 
 			mbCounts := map[int64]store.MailboxCounts{}
+			var totalSize int64
 			err = bstore.QueryDB[store.Message](ctxbg, db).ForEach(func(m store.Message) error {
 				mb := mailboxes[m.MailboxID]
 				if m.UID >= mb.UIDNext {
@@ -282,6 +283,7 @@ possibly making them potentially no longer readable by the previous version.
 				if m.Expunged {
 					return nil
 				}
+				totalSize += m.Size
 
 				mp := store.MessagePath(m.ID)
 				seen[mp] = struct{}{}
@@ -317,12 +319,28 @@ possibly making them potentially no longer readable by the previous version.
 			})
 			checkf(err, dbpath, "reading messages in account database to check files")
 
+			haveCounts := true
 			for _, mb := range mailboxes {
 				// We only check if database doesn't have zero values, i.e. not yet set.
+				if !mb.HaveCounts {
+					haveCounts = false
+				}
 				if mb.HaveCounts && mb.MailboxCounts != mbCounts[mb.ID] {
 					checkf(errors.New(`wrong mailbox counts, see "mox recalculatemailboxcounts"`), dbpath, "mailbox %q (id %d) has wrong counts %s, should be %s", mb.Name, mb.ID, mb.MailboxCounts, mbCounts[mb.ID])
 				}
 			}
+
+			if haveCounts {
+				du := store.DiskUsage{ID: 1}
+				err := db.Get(ctxbg, &du)
+				if err == nil {
+					if du.MessageSize != totalSize {
+						checkf(errors.New(`wrong total message size, see mox recalculatemailboxcounts"`), dbpath, "account has wrong total message size %d, should be %d", du.MessageSize, totalSize)
+					}
+				} else if err != nil && !errors.Is(err, bstore.ErrAbsent) {
+					checkf(err, dbpath, "get disk usage")
+				}
+			}
 		}
 
 		// Walk through all files in the msg directory. Warn about files that weren't in