Send the build version in the outbound User-Agent (closes #313)

The http and slack targets sent the fixed User-Agent webhooker/1.0. The
delivery engine now takes the version from Globals, the value the web UI
footer shows, and one method builds webhooker/<version> for both
targets. The http target still sets it after the configured headers, so
a configured or inbound User-Agent never reaches the wire.

A test builds the engine through its real constructor with a known
version and checks the header each target sends.

Model: opus-5-5

internal/delivery/redirect_test.go

@@ -375,6 +375,7 @@ func TestApplyRequestHeaders_ReportsOriginScopedNames(t *testing.T) {
 				"Content-Type":  testContentType,
 			},
 		},
+		"webhooker/dev",
 	)
 
 	assert.Equal(t,