feat(rda): measure the two district sources before arbitrating between them

A log imported from HAMLOG.online carries the Russian district in CNTY, and the
offline RDA database has its own answer for the same contact. Which wins is a
real question — but only if the two are independent. If HAMLOG's value is itself
a lookup in the same kind of database, there is nothing to arbitrate and a
precedence rule would be ceremony around a redundancy.

So this measures instead of deciding: Settings → RDA gains a read-only compare
that reports agreements, disagreements, and the contacts where only one side
knows, then lists the disagreements — callsign, date, both districts, whether
the database answer was a DATED record (the award administrators' own fact) or
the callsign's current district (an assumption), and whether the QSO is
confirmed on HAMLOG (their value has then been through a match of two logs).

It changes nothing in the log. What to do about a conflict is the next decision,
and it should be taken with the operator's own numbers in hand.

Also: the four HAMLOG fields become filterable, so 'never sent there' is a
filter rather than a guess. A field that can be written and not read back is
half a feature — pinned by a test against the bulk-editable list.

internal/qso/filter_in_test.go

@@ -55,3 +55,24 @@ func TestNotInKeepsNulls(t *testing.T) {
 		t.Errorf("sql = %q, want the column wrapped in IFNULL", sql)
 	}
 }
+
+// Every HAMLOG field an operator can bulk-edit must also be filterable: the
+// question "which contacts have never gone to HAMLOG" is what you ask BEFORE
+// uploading a backlog, and a field that can be written but not read back is
+// half a feature.
+func TestHamlogFieldsAreFilterable(t *testing.T) {
+	filterable := map[string]bool{}
+	for _, f := range FilterableFields() {
+		filterable[f] = true
+	}
+	for _, f := range []string{"hamlog_sent", "hamlog_sent_date", "hamlog_rcvd", "hamlog_rcvd_date"} {
+		if !filterable[f] {
+			t.Errorf("%s is bulk-editable but not filterable", f)
+		}
+	}
+	// The received key must be THEIR name, or an imported log filters differently
+	// from a stamped one — the exact split the confirmation source exists to avoid.
+	if got := filterableExtras["hamlog_rcvd"]; got != "APP_HAMLOG_QSO_CFM" {
+		t.Errorf("hamlog_rcvd filters on %q, want APP_HAMLOG_QSO_CFM", got)
+	}
+}

internal/qso/qso.go

@@ -1316,6 +1316,15 @@ var bareDateRe = regexp.MustCompile(`^\d{4}-\d{2}-\d{2}$`)
 // ADIF/Extras key; the SQL expression uses json_extract.
 var filterableExtras = map[string]string{
 	"owner_callsign": "OWNER_CALLSIGN",
+	// HAMLOG.online, in both directions and with its dates. Filterable for the
+	// same reason it is bulk-editable: without it there is no way to ask "which
+	// contacts have never gone there", which is the question an operator asks
+	// before uploading a backlog. The received key is THEIR field name — see
+	// award.HamlogQSLKey — so an imported log filters the same as a stamped one.
+	"hamlog_sent":      "APP_OPSLOG_HAMLOG_SENT",
+	"hamlog_sent_date": "APP_OPSLOG_HAMLOG_SENT_DATE",
+	"hamlog_rcvd":      "APP_HAMLOG_QSO_CFM",
+	"hamlog_rcvd_date": "APP_OPSLOG_HAMLOG_QSL_DATE",
 }
 
 // FilterableFields returns the whitelist (for the frontend to build its field