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>

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)
 # ============================================================