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/import.go

@@ -464,10 +464,17 @@ func recordToQSO(rec Record) (qso.QSO, bool) {
 	q.QSLRcvd = rec["qsl_rcvd"]
 	q.QSLSentDate = rec["qslsdate"]
 	q.QSLRcvdDate = rec["qslrdate"]
+	// QSL_VIA is the manager. QSL_SENT_VIA / QSL_RCVD_VIA are the routing
+	// method, an enumeration of their own.
+	//
+	// These used to be one field here: an empty QSL_VIA was filled from
+	// QSL_SENT_VIA, on the theory that loggers writing one meant the other.
+	// They do not — Log4OM defaults QSL_SENT_VIA to E, and the import put "E"
+	// where every panel in OpsLog shows the manager's callsign. Keeping them
+	// apart is also what lets an export give them back.
 	q.QSLVia = rec["qsl_via"]
-	if q.QSLVia == "" { // many loggers (Log4OM) write QSL_SENT_VIA instead
-		q.QSLVia = rec["qsl_sent_via"]
-	}
+	q.QSLSentVia = NormaliseQSLVia(rec["qsl_sent_via"])
+	q.QSLRcvdVia = NormaliseQSLVia(rec["qsl_rcvd_via"])
 	q.QSLMsg = rec["qslmsg"]
 	q.QSLMsgRcvd = rec["qslmsg_rcvd"]
 	q.LOTWSent = rec["lotw_qsl_sent"]