fix(httpapi): GetGraph 500s when rel_type is omitted

req.Params.RelType is *[]string; passing the nil pointer straight
through as a pgx query arg (both in the blast_radius() call and in
ListGraphEdges) panics because pgx can't infer the array element type
from a nil *[]string, only from a concrete (possibly nil) []string.
Dereference once up front instead. Also affected the sqlc-based
ListGraphEdges path added by the R3 refactor, which had the same bug.

Add a regression test for GET /api/v1/graph?root=X&depth=N with no
rel_type.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/httpapi/api_test.go

@@ -263,6 +263,20 @@ func TestAPIEndToEnd(t *testing.T) {
 		}
 	})
 
+	// Regression: rel_type is an optional array param (*[]string); when
+	// omitted entirely (not an empty list), passing the nil pointer straight
+	// through to pgx as a query arg panics because pgx can't infer the array
+	// element type from a nil *[]string. root+depth alone must still work.
+	t.Run("graph without rel_type", func(t *testing.T) {
+		rec, body := get(t, h, "/api/v1/graph?root=host:hubris&depth=1", nil)
+		if rec.Code != 200 {
+			t.Fatalf("status %d", rec.Code)
+		}
+		if len(body["nodes"].([]any)) < 2 {
+			t.Errorf("graph too small: %d nodes", len(body["nodes"].([]any)))
+		}
+	})
+
 	t.Run("ontology", func(t *testing.T) {
 		rec, body := get(t, h, "/api/v1/ontology", nil)
 		if rec.Code != 200 {