feat(smtp_relay): move probe forwarding to realism worker via bus

Attacker probe emails are now forwarded by the master (realism worker)
rather than inside the MACVLAN container, which has no internet gateway.

- New smtp.probe.pending bus topic: ingester publishes when smtp_relay
  message_stored fires; worker subscribes and does the actual delivery
- decnet/orchestrator/drivers/smtp_relay.py: pure-sync forward_probe()
  reads the .eml from disk and sends via smtplib on a thread executor
- worker.py: _run_smtp_probe_listener + _handle_probe_pending subtask;
  limit enforced via count_probe_relays() (DB-backed, restart-safe)
- bounties.py: count_probe_relays() query on probe_relay bounty type
- fleet.py: get_fleet_decky_by_name() to pull service config from DB
- services/smtp_relay.py: upstream_* and probe_limit fields defined in
  config_schema but NOT injected into container env (credentials stay
  out of docker env vars)
- ingester.py: stripped of smtplib; publishes probe.pending and exits
- tests: assert upstream keys absent from container environment

decnet/bus/topics.py

@@ -54,6 +54,7 @@ SYSTEM = "system"
 CREDENTIAL = "credential"
 ORCHESTRATOR = "orchestrator"
 CANARY = "canary"
+SMTP = "smtp"
 
 
 # ─── Leaf event-type constants (the last segment of each topic) ──────────────
@@ -394,6 +395,16 @@ def system_control(worker: str) -> str:
     return f"{SYSTEM}.{worker}.{SYSTEM_CONTROL}"
 
 
+def smtp(event_type: str) -> str:
+    """Build ``smtp.<event_type>``.
+
+    *event_type* may contain dots (e.g. ``probe.pending``).
+    """
+    if not event_type:
+        raise ValueError("smtp topic requires a non-empty event_type")
+    return f"{SMTP}.{event_type}"
+
+
 def _reject_tokens(*parts: str) -> None:
     """Reject topic segments that would break NATS-style tokenization.