feat: add smtp_relay service; add service_testing/ init

- decnet/services/smtp_relay.py: open relay variant of smtp, same template
  with SMTP_OPEN_RELAY=1 baked into the environment
- tests/service_testing/__init__.py: init so pytest discovers the subdirectory

decnet/services/smtp_relay.py

@@ -0,0 +1,43 @@
+from pathlib import Path
+
+from decnet.services.base import BaseService
+
+# Reuses the same template as the smtp service — only difference is
+# SMTP_OPEN_RELAY=1 in the environment, which enables the open relay persona.
+_TEMPLATES_DIR = Path(__file__).parent.parent.parent / "templates" / "smtp"
+
+
+class SMTPRelayService(BaseService):
+    """SMTP open relay bait — accepts any RCPT TO and delivers messages."""
+
+    name = "smtp_relay"
+    ports = [25, 587]
+    default_image = "build"
+
+    def compose_fragment(
+        self,
+        decky_name: str,
+        log_target: str | None = None,
+        service_cfg: dict | None = None,
+    ) -> dict:
+        cfg = service_cfg or {}
+        fragment: dict = {
+            "build": {"context": str(_TEMPLATES_DIR)},
+            "container_name": f"{decky_name}-smtp_relay",
+            "restart": "unless-stopped",
+            "cap_add": ["NET_BIND_SERVICE"],
+            "environment": {
+                "NODE_NAME": decky_name,
+                "SMTP_OPEN_RELAY": "1",
+            },
+        }
+        if log_target:
+            fragment["environment"]["LOG_TARGET"] = log_target
+        if "banner" in cfg:
+            fragment["environment"]["SMTP_BANNER"] = cfg["banner"]
+        if "mta" in cfg:
+            fragment["environment"]["SMTP_MTA"] = cfg["mta"]
+        return fragment
+
+    def dockerfile_context(self) -> Path:
+        return _TEMPLATES_DIR