feat(cluster): a SOTA column, and a spot click that credits the summit

The SOTA feeds put the summit in the spot's own text, so there is nothing to
poll: a regex on the comment gives the reference, and clicking the spot writes
SOTA@<ref> into the award references the way a park already wrote POTA@<ref>.

The regex is narrow on purpose — association/region-NNN, anchored — because
POTA (US-4475), WWFF (DLFF-0001) and portable callsigns (DL/SP9DPM/P) share
that field and none of them is a summit. Pinned by a table test.

Off by default: the reference is only there on the summit feeds, and an empty
column for every operator who does not chase them is worse than a checkbox.

internal/cluster/sotaref_test.go

@@ -0,0 +1,25 @@
+package cluster
+
+import "testing"
+
+func TestSOTARefFrom(t *testing.T) {
+	// Left column: real comments seen on the SOTA cluster feed.
+	cases := []struct{ in, want string }{
+		{"W9/WI-001", "W9/WI-001"},
+		{"DM/BM-063", "DM/BM-063"},
+		{"W7Y/TT-122", "W7Y/TT-122"},
+		{"VK3/VC-014 s2s", "VK3/VC-014"},
+		{"[SOTA] F/AM-123 cq", "F/AM-123"},
+		{"", ""},
+		// The other reference schemes that share this field.
+		{"POTA US-4475", ""},
+		{"WWFF DLFF-0001", ""},
+		// A portable callsign is not a summit.
+		{"DL/SP9DPM/P calling", ""},
+	}
+	for _, c := range cases {
+		if got := SOTARefFrom(c.in); got != c.want {
+			t.Errorf("SOTARefFrom(%q) = %q, want %q", c.in, got, c.want)
+		}
+	}
+}