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

The http and slack targets sent the constant User-Agent webhooker/1.0. They now send webhooker/ followed by the version the build stamped, the same value the footer shows, built in one place on the delivery engine. User-Agent stays a reserved header and is still set after the target's configured headers, so a configured one cannot override it. Tests check the header each target sends against a known version, and that a configured User-Agent is replaced.

Model: opus-5-5

internal/delivery/export_test.go

@@ -83,8 +83,9 @@ func ExportApplyRequestHeaders(
 	req *http.Request,
 	event *database.Event,
 	cfg *HTTPTargetConfig,
+	userAgent string,
 ) []string {
-	return applyRequestHeaders(req, event, cfg)
+	return applyRequestHeaders(req, event, cfg, userAgent)
 }
 
 // ExportTruncate exposes truncate for testing.