quota: Fix maildir quota drop to zero after IMAP MOVE

[LexxFedoroff]

Problem

When mailbox_move() is called with the maildir quota driver, the source
mail's expunge is double-counted, causing reported quota to drop to 0 after
any IMAP MOVE operation.

Symptom

# Before move – correct
$ doveadm quota get -u user@example.com
Quota name  Type     Value  Limit   %
user        STORAGE   1387   5120  27

# After IMAP MOVE to another folder – broken
$ doveadm quota get -u user@example.com
Quota name  Type     Value  Limit   %
user        STORAGE      0   5120   0

The message physically exists in the destination folder; only the quota tracking
is wrong. The maildirsize file accumulates an unbalanced negative delta with no
corresponding positive entry.

Root Cause

Two separate code paths both account for the source mail's expunge:

1. Destination transaction — quota_save_finish → quota_check →
quota_try_alloc:

quota_alloc_with_size(ctx, size=N, expunged_size=N);
//   ctx->bytes_used += N    (save to destination)
//   ctx->bytes_used -= N    (quota_used_apply_expunged subtracts expunged_size)
//   net: ctx->bytes_used = 0  → nothing written to maildirsize

2. Source transaction — mail_expunge → quota_mail_expunge →
quota_free_bytes:

// Always fires, regardless of ctx->moving
src_qt->bytes_used -= N;   // → writes "-N -1" to maildirsize

Result: maildirsize receives only the negative delta; the positive entry for
the destination is never written.

Fix

Add a moving flag to quota_transaction_context. It is set from
mail_save_context.moving in quota_check(). In quota_try_alloc(), pass
expunged_size=0 to quota_alloc_with_size() when ctx->moving is TRUE.

expunged_size remains unchanged in the quota_test_alloc() call, so moves at
the quota boundary are still correctly permitted. The actual accounting for the
source expunge continues to be handled by the source transaction's
quota_mail_expunge() -> quota_free_bytes() path.

After the fix:

Transaction	bytes_used	Written to maildirsize
Destination	+N	+N +1
Source	-N	-N -1
Net	0	correct

Testing

Reproduce with the maildir quota driver:

doveadm quota recalc -u user@example.com
doveadm move -u user@example.com INBOX.Drafts mailbox INBOX ALL
doveadm quota get -u user@example.com   # was 0 before this fix

COPY + EXPUNGE is unaffected (the moving flag remains FALSE in that path).

Files changed

• src/plugins/quota/quota-private.h — add moving:1 bitfield to
  quota_transaction_context
• src/plugins/quota/quota-storage.c — propagate ctx->moving into the quota
  transaction in quota_check()
• src/plugins/quota/quota.c — skip subtracting expunged_size from
  bytes_used in quota_try_alloc() when the transaction is a MOVE