fix: count events at receipt, not broadcast, to match Go relay semantics
relay_frames_received_total was only incremented in broadcast(), missing
events dropped by inactive-account filtering or validation failure.
move the counter to subscriber's serverMessage after frame decode, matching
the Go relay's events_received_counter which counts at processRepoEvent entry.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>