feat(email): add provider abstraction and HTML templates

Add pluggable email provider system supporting SMTP, Mailgun, and SendGrid
with factory pattern for runtime provider selection. Includes branded HTML
email templates for verification, password reset, MFA notifications, and
organization invites.

Also rebrands all email content from Gatehouse to Secuird, adds email
provider configuration options, and fixes duplicate log handlers in
development mode.

gatehouse_app/services/providers/mailgun_provider.py

@@ -0,0 +1,83 @@
+"""Mailgun email provider implementation."""
+import logging
+
+import requests
+from flask import current_app
+
+from gatehouse_app.services.email_provider import EmailMessage, EmailProvider
+
+logger = logging.getLogger(__name__)
+
+
+class MailgunEmailProvider(EmailProvider):
+    """Mailgun API-based email provider implementation."""
+
+    # Configuration keys
+    MAILGUN_API_KEY = "MAILGUN_API_KEY"
+    MAILGUN_DOMAIN = "MAILGUN_DOMAIN"
+    MAILGUN_API_URL = "MAILGUN_API_URL"
+    FROM_ADDRESS = "FROM_ADDRESS"
+
+    DEFAULT_API_URL = "https://api.mailgun.net/v3"
+
+    def send(self, message: EmailMessage) -> bool:
+        """Send an email via Mailgun API.
+
+        Args:
+            message: EmailMessage instance containing email details
+
+        Returns:
+            bool: True if email was sent successfully, False otherwise
+        """
+        api_key = current_app.config.get(self.MAILGUN_API_KEY)
+        domain = current_app.config.get(self.MAILGUN_DOMAIN)
+        api_url = current_app.config.get(self.MAILGUN_API_URL, self.DEFAULT_API_URL)
+        default_from = current_app.config.get(self.FROM_ADDRESS)
+
+        missing = [k for k, v in [("MAILGUN_API_KEY", api_key), ("MAILGUN_DOMAIN", domain)] if not v]
+        if missing:
+            logger.error(
+                f"[MAILGUN] Cannot send — missing config: {', '.join(missing)}. "
+                f"Would have sent to: {message.to} | Subject: {message.subject}"
+            )
+            return False
+
+        from_address = message.from_address or default_from
+        if not from_address:
+            logger.error(
+                f"[MAILGUN] Cannot send — missing FROM_ADDRESS. "
+                f"Would have sent to: {message.to} | Subject: {message.subject}"
+            )
+            return False
+
+        url = f"{api_url}/{domain}/messages"
+
+        data = {
+            "to": message.to,
+            "subject": message.subject,
+            "text": message.body,
+            "from": from_address,
+        }
+        if message.html_body:
+            data["html"] = message.html_body
+
+        try:
+            response = requests.post(
+                url,
+                auth=("api", api_key),
+                data=data,
+            )
+
+            if response.status_code == 200:
+                logger.info(f"[MAILGUN] Sent to {message.to} | Subject: {message.subject}")
+                return True
+            else:
+                logger.error(
+                    f"[MAILGUN] Failed to send to {message.to}: from {from_address}"
+                    f"status={response.status_code} body={response.text}"
+                )
+                return False
+
+        except Exception as e:
+            logger.error(f"[MAILGUN] Exception while sending to {message.to}: {e}")
+            return False