fix(qsl): separate the QSL manager from the routing method (#16)

QSL_VIA is the manager. QSL_SENT_VIA and QSL_RCVD_VIA are the ADIF "QSL Via"
enumeration — B bureau, D direct, E electronic, M manager (import-only) —
and say how a card travelled. OpsLog had one column for all three:

  - the import folded QSL_SENT_VIA into QSL_VIA whenever QSL_VIA was empty,
    which is exactly a Log4OM export (it defaults QSL_SENT_VIA to E), so
    OE6CLD saw "E" everywhere OpsLog shows the manager;
  - QSL_RCVD_VIA was listed in adifPromoted with no column behind it, so it
    was not stored, not kept among the extras, and not exported — dropped
    outright on import;
  - neither was ever written on export, so an import followed by an export
    destroyed both;
  - and OpsLog polluted the field itself: the QSL Manager panel wrote
    "Bureau" / "Direct" / "Electronic", in full words, into QSL_VIA.

Two columns added (migration 0027), carried through the five places a
promoted ADIF field has to touch, with round-trip tests pinning the reported
case. The QSL panel now offers Bureau / Direct / Electronic for each
direction and stores the enumeration; the manager field is labelled as the
manager and holds only that. M is kept when a file gives it and never
written back out.

Existing logs hold a mixture of the two in one column. The repair is offered,
not performed: the count is shown once per log with a plain question, and a
"no" is remembered. It moves only where QSL_SENT_VIA is still empty, and only
values that normalise to the enumeration — a manager is a callsign and can
never be one of those six words, which a test pins against real manager calls.

internal/adif/export.go

@@ -241,6 +241,14 @@ func writeRecord(bw *bufio.Writer, q qso.QSO, includeApp bool, allow map[string]
 	w("QSLSDATE", q.QSLSentDate)
 	w("QSLRDATE", q.QSLRcvdDate)
 	w("QSL_VIA", q.QSLVia)
+	// M (manager) is import-only in the QSL Via enumeration: we keep it when a
+	// file gives it to us, but a file we write must not carry it.
+	if q.QSLSentVia != QSLViaManager {
+		w("QSL_SENT_VIA", q.QSLSentVia)
+	}
+	if q.QSLRcvdVia != QSLViaManager {
+		w("QSL_RCVD_VIA", q.QSLRcvdVia)
+	}
 	w("QSLMSG", q.QSLMsg)
 	w("QSLMSG_RCVD", q.QSLMsgRcvd)
 	w("LOTW_QSL_SENT", q.LOTWSent)