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>

email_template.txt

@@ -0,0 +1,11 @@
+Hello,
+
+Your OpenVPN configuration has been updated for: {cn}
+
+Retrieve your one-time password here (link expires after one view):
+{url}
+
+Your configuration file is attached as '{config_name}'.
+
+Regards,
+VPN Admin

openvpncertupdate.py

@@ -309,6 +309,50 @@ def create_note(content: str, base_url: str) -> str:
     return f"{base_url.rstrip('/')}/#/note/{note_id}/{key_fragment}"
 
 
+# ============================================================
+# === MAILER ===
+# ============================================================
+
+
+def send_email(
+    to_address: str,
+    cn: str,
+    one_time_url: str,
+    ovpn_path: str,
+    mail_from: str,
+    subject: str,
+    template_path: str,
+    mail_binary: str,
+) -> None:
+    """Compose and send email with .ovpn attachment via msmtp/sendmail."""
+    config_name = os.path.basename(ovpn_path)
+    body = Path(template_path).read_text().format(
+        cn=cn, url=one_time_url, config_name=config_name,
+    )
+    msg = email.mime.multipart.MIMEMultipart()
+    msg["From"]    = mail_from
+    msg["To"]      = to_address
+    msg["Subject"] = subject
+    msg.attach(email.mime.text.MIMEText(body, "plain"))
+
+    with open(ovpn_path, "rb") as fh:
+        part = email.mime.base.MIMEBase("application", "octet-stream")
+        part.set_payload(fh.read())
+    email.encoders.encode_base64(part)
+    part.add_header("Content-Disposition", f'attachment; filename="{config_name}"')
+    msg.attach(part)
+
+    proc = subprocess.Popen(
+        [mail_binary, "-t"],
+        stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE,
+    )
+    _, err = proc.communicate(input=msg.as_bytes())
+    if proc.returncode != 0:
+        raise RuntimeError(
+            f"mail delivery failed (exit {proc.returncode}): {err.decode().strip()}"
+        )
+
+
 # (remaining sections added in later tasks)
 # ============================================================
 

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")