feat: wire SendGridEmailBackend with from_email/reply_to settings

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

backend/tests/test_email_backend.py

@@ -0,0 +1,40 @@
+"""Unit tests for SendGridEmailBackend — no Postgres needed."""
+import pytest
+
+
+def test_sendgrid_send_calls_api(monkeypatch):
+    calls = []
+
+    class FakeResponse:
+        status_code = 202
+        body = b""
+
+    class FakeClient:
+        def send(self, msg):
+            calls.append(msg)
+            return FakeResponse()
+
+    monkeypatch.setattr("app.services.email.SendGridAPIClient", lambda _key: FakeClient())
+
+    from app.services.email import SendGridEmailBackend
+    backend = SendGridEmailBackend()
+    backend.send(to="to@example.com", subject="Hello", html="<p>Hi</p>")
+
+    assert len(calls) == 1
+
+
+def test_sendgrid_raises_on_4xx(monkeypatch):
+    class FakeResponse:
+        status_code = 400
+        body = b"bad request"
+
+    class FakeClient:
+        def send(self, msg):
+            return FakeResponse()
+
+    monkeypatch.setattr("app.services.email.SendGridAPIClient", lambda _key: FakeClient())
+
+    from app.services.email import SendGridEmailBackend
+    backend = SendGridEmailBackend()
+    with pytest.raises(RuntimeError, match="400"):
+        backend.send(to="to@example.com", subject="Hello", html="<p>Hi</p>")