feat(filter): filter on when a QSO was added to the log

After an import that inserted instead of updating, there was no way to
ask which records it created: an import of old contacts carries old QSO
dates, so it hides among them. created_at is the only column that knows,
and it was not filterable.

Listed as a date column so a bare YYYY-MM-DD compares on the date part,
like qso_date.

internal/qso/qso.go

@@ -1291,11 +1291,16 @@ var filterableColumns = map[string]bool{
 	"my_wwff_ref": true, "my_street": true, "my_city": true, "my_postal_code": true,
 	"my_rig": true, "my_antenna": true, "my_sig": true, "my_sig_info": true,
 	"tx_pwr": true, "comment": true, "notes": true,
+	// When the record entered THIS logbook, as opposed to when the contact was
+	// made. The question it answers is "what did that import actually add?" —
+	// unanswerable otherwise, since an import of old contacts carries old QSO
+	// dates and hides inside the log.
+	"created_at": true,
 }
 
 // dateColumns are stored as full ISO timestamps; a filter on a bare YYYY-MM-DD
 // value compares on the date part (see conditionSQL) so day filters are exact.
-var dateColumns = map[string]bool{"qso_date": true, "qso_date_off": true}
+var dateColumns = map[string]bool{"qso_date": true, "qso_date_off": true, "created_at": true}
 
 // numericColumns are the filterable columns holding numbers rather than text.
 //