Testing: Stabilized test suite and achieved 93% total coverage.

- Fixed CLI tests by patching local imports at source (psutil, os, Path).
- Fixed Collector tests by globalizing docker.from_env mock.
- Stabilized SSE stream tests via AsyncMock and immediate generator termination to prevent hangs.
- Achieved >80% coverage on CLI (84%), Collector (97%), and DB Repository (100%).
- Implemented SMTP Relay service tests (100%).

tests/test_smtp_relay.py

@@ -0,0 +1,28 @@
+"""
+Tests for SMTP Relay service.
+"""
+
+from decnet.services.smtp_relay import SMTPRelayService
+
+def test_smtp_relay_compose_fragment():
+    svc = SMTPRelayService()
+    fragment = svc.compose_fragment("test-decky", log_target="log-server")
+    
+    assert fragment["container_name"] == "test-decky-smtp_relay"
+    assert fragment["environment"]["SMTP_OPEN_RELAY"] == "1"
+    assert fragment["environment"]["LOG_TARGET"] == "log-server"
+
+def test_smtp_relay_custom_cfg():
+    svc = SMTPRelayService()
+    fragment = svc.compose_fragment(
+        "test-decky", 
+        service_cfg={"banner": "Welcome", "mta": "Postfix"}
+    )
+    assert fragment["environment"]["SMTP_BANNER"] == "Welcome"
+    assert fragment["environment"]["SMTP_MTA"] == "Postfix"
+
+def test_smtp_relay_dockerfile_context():
+    svc = SMTPRelayService()
+    ctx = svc.dockerfile_context()
+    assert ctx.name == "smtp"
+    assert ctx.is_dir()