feat(smtp_relay): ingest probe_forwarded as probe_relay bounty

Adds probe_forwarded to meaningful event kinds and stores it in the
bounty table as bounty_type=probe_relay with forwarded=true/false, so
the dashboard shows whether the upstream actually accepted the test email.

decnet/correlation/event_kinds.py

@@ -41,6 +41,10 @@ INTERACTION_EVENT_TYPES: frozenset[str] = frozenset({
     "rcpt_to",
     "rcpt_denied",
     "message_accepted",
+    # probe_forwarded fires when we actually relay the test email upstream
+    # so the attacker can verify receipt. forwarded=1 means the upstream
+    # accepted it; forwarded=0 means it failed (but the attacker still got 250).
+    "probe_forwarded",
     # File / payload activity
     "file_captured",
     "upload",