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/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