feat(slice-2): platform/mailer port + LogMailer (dev channel, §6.2/PUC-10)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/tests/test_mailer.py

@@ -0,0 +1,26 @@
+from app.platform import mailer
+
+
+def test_logmailer_captures_to_outbox():
+    m = mailer.LogMailer()
+    assert m.outbox == []
+    m.send("merchant@example.com", "Your ecomm code: 123456", "Code: 123456 (valid 10 min)")
+    assert len(m.outbox) == 1
+    msg = m.outbox[-1]
+    assert msg.to == "merchant@example.com"
+    assert msg.subject == "Your ecomm code: 123456"
+    assert "123456" in msg.body
+
+
+def test_build_mailer_log_kind():
+    m = mailer.build_mailer("log")
+    assert isinstance(m, mailer.LogMailer)
+
+
+def test_build_mailer_smtp_not_yet_available():
+    # SmtpMailer lands in SLICE-4; until then asking for it is an explicit error, not a
+    # silent fallback to LogMailer (which would send no real mail in a deployed env).
+    import pytest
+
+    with pytest.raises(NotImplementedError):
+        mailer.build_mailer("smtp")