feat(log): QSO number column, oldest contact = 1

Not the id: the primary key follows insertion order, so importing an old ADIF
gives the oldest contacts the highest ids. This is a rank over qso_date.

Computed over the WHOLE log, not the query result — ranking inside the result
would renumber every contact the moment a filter is applied, and QSO #1 would
change identity as the operator typed.

Held as one id-to-rank map, built from a single ordered id query and dropped
with the other derived indexes when the log changes. A contact logged from the
entry form is the newest, so it takes the next number without rereading the
log: AddQSO deliberately avoids full invalidation because a contest run would
pay for it once per QSO. A contact entered with an OLDER date belongs in the
middle of the order, so there the map is dropped and rebuilt rather than
mis-numbered.

internal/qso/qso.go

@@ -47,7 +47,15 @@ func MergeNonZero(dst *QSO, src QSO) {
 // import/export. Pointers are used to distinguish "absent" from "zero".
 // Anything in ADIF that is not a promoted column lands in Extras.
 type QSO struct {
-	ID         int64     `json:"id"`
+	ID int64 `json:"id"`
+	// Number is the contact's position in the log, oldest = 1.
+	//
+	// NOT a column and NOT the ID: the primary key follows insertion order, so
+	// importing an old ADIF gives the oldest contacts the highest ids. This is a
+	// rank over qso_date, computed by the App layer and stamped on the way out —
+	// which is why it is `json:"-"`-adjacent in spirit: nothing reads or writes
+	// it in SQL.
+	Number     int       `json:"number,omitempty"`
 	Callsign   string    `json:"callsign"`
 	QSODate    time.Time `json:"qso_date"`               // start, UTC
 	QSODateOff time.Time `json:"qso_date_off,omitempty"` // end, UTC
@@ -3170,3 +3178,32 @@ func (r *Repo) WorkedGridKeys(ctx context.Context, normMode func(string) string)
 	}
 	return out, rows.Err()
 }
+
+// OrderedIDs returns every QSO id in chronological order, oldest first.
+//
+// The mirror of ListFiltered's "qso_date DESC, id DESC", so the tie-break is the
+// same one the grid displays and a contact cannot change number depending on
+// which way it is read.
+//
+// One query, ids only: the caller turns it into a rank once and keeps it in
+// memory. Asking the database for a row's rank per row would be a correlated
+// count over the whole log for each of thirty thousand rows.
+func (r *Repo) OrderedIDs(ctx context.Context) ([]int64, time.Time, error) {
+	rows, err := r.db.QueryContext(ctx, `SELECT id, qso_date FROM qso ORDER BY qso_date ASC, id ASC`)
+	if err != nil {
+		return nil, time.Time{}, err
+	}
+	defer rows.Close()
+	out := make([]int64, 0, 4096)
+	var newest time.Time
+	for rows.Next() {
+		var id int64
+		var at time.Time
+		if err := rows.Scan(&id, &at); err != nil {
+			return nil, time.Time{}, err
+		}
+		out = append(out, id)
+		newest = at // ascending, so the last row read is the newest
+	}
+	return out, newest, rows.Err()
+}