feat: MIME email composer with .ovpn attachment via msmtp/sendmail

Add send_email() function that composes multipart MIME messages with
template-based body and .ovpn file attachment, piped to mail binary via
subprocess.Popen. Supports Cryptgeon URL insertion and sender metadata.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_mailer.py

@@ -0,0 +1,47 @@
+import pytest
+from unittest.mock import patch, MagicMock
+from openvpncertupdate import send_email
+
+
+def make_popen(returncode=0):
+    proc = MagicMock()
+    proc.communicate.return_value = (b"", b"")
+    proc.returncode = returncode
+    return proc
+
+
+def test_calls_mail_binary(tmp_path):
+    tmpl = tmp_path / "t.txt"; tmpl.write_text("{cn} {url} {config_name}")
+    att  = tmp_path / "c.ovpn"; att.write_text("x")
+    captured = {}
+    def fake_popen(cmd, **kw): captured["cmd"] = cmd; return make_popen()
+    with patch("openvpncertupdate.subprocess.Popen", side_effect=fake_popen):
+        send_email("to@x.com", "alice", "http://u", str(att),
+                   "f@x.com", "Subj", str(tmpl), "msmtp")
+    assert captured["cmd"] == ["msmtp", "-t"]
+
+
+def test_body_contains_cn_and_url(tmp_path):
+    tmpl = tmp_path / "t.txt"; tmpl.write_text("Dear {cn}, see {url} for {config_name}")
+    att  = tmp_path / "c.ovpn"; att.write_text("")
+    received = {}
+    def fake_popen(cmd, **kw):
+        p = MagicMock()
+        p.communicate = lambda input=None: (received.update({"data": input}) or b"", b"")
+        p.returncode = 0
+        return p
+    with patch("openvpncertupdate.subprocess.Popen", side_effect=fake_popen):
+        send_email("to@x.com", "bob", "http://secret", str(att),
+                   "f@x.com", "s", str(tmpl), "msmtp")
+    body = received["data"].decode()
+    assert "bob" in body
+    assert "http://secret" in body
+
+
+def test_raises_on_failure(tmp_path):
+    tmpl = tmp_path / "t.txt"; tmpl.write_text("{cn} {url} {config_name}")
+    att  = tmp_path / "c.ovpn"; att.write_text("")
+    with patch("openvpncertupdate.subprocess.Popen", return_value=make_popen(returncode=1)):
+        with pytest.raises(RuntimeError, match="mail"):
+            send_email("x@y.com", "cn", "url", str(att),
+                       "f@x.com", "s", str(tmpl), "msmtp")