feat(organizations): email inviter when membership invite is accepted

When a user accepts an org invite, send a notification email to the
person who sent the invite with membership details (member name, email,
org name, role) and an optional View Organization button.

Added build_invite_accepted_html() template to email_templates.py,
wired it into the accept_invite() handler, and added a test case.

gatehouse_app/services/email_templates.py

@@ -562,3 +562,51 @@ def build_contact_enquiry_html(
     <p style="margin: 0; color: {TEXT_COLOR}; font-size: 14px; line-height: 1.6; white-space: pre-wrap;">{message_display}</p>
     '''
     return get_base_html(content, f"Secuird Website: {type_label}", f"New {type_label} from {submitter_email}")
+
+
+def build_invite_accepted_html(
+    inviter_name: str,
+    member_name: str,
+    member_email: str,
+    org_name: str,
+    role: str,
+    org_link: Optional[str] = None,
+) -> str:
+    """Build invite accepted notification email.
+
+    Args:
+        inviter_name: Name of the person who sent the invite
+        member_name: Name of the person who accepted
+        member_email: Email of the person who accepted
+        org_name: Organization name
+        role: Role assigned to the member
+        org_link: Optional link to view the organization
+
+    Returns:
+        HTML email string
+    """
+    content = f'''
+    <h2 style="margin: 0 0 20px 0; color: {TEXT_COLOR}; font-size: 20px; font-weight: 600;">Invitation Accepted</h2>
+    <p style="margin: 0 0 20px 0; color: {TEXT_COLOR}; font-size: 15px; line-height: 1.6;">
+        <strong>{member_name}</strong> has accepted your invitation to join <strong>{org_name}</strong> on Secuird.
+    </p>
+    {get_alert_box(f"<strong>{member_name}</strong> ({member_email}) has joined <strong>{org_name}</strong>", "success", "✅")}
+    <table role="presentation" width="100%" cellspacing="0" cellpadding="0" style="margin: 20px 0; background-color: {BACKGROUND_COLOR}; border-radius: 8px;">
+        <tr>
+            <td style="padding: 20px;">
+                <h3 style="margin: 0 0 16px 0; color: {TEXT_COLOR}; font-size: 14px; font-weight: 600;">Membership Details</h3>
+                <table role="presentation" width="100%" cellspacing="0" cellpadding="0">
+                    {get_detail_row("Member", member_name)}
+                    {get_detail_row("Email", member_email)}
+                    {get_detail_row("Organization", org_name)}
+                    {get_detail_row("Role", role)}
+                </table>
+            </td>
+        </tr>
+    </table>
+    '''
+    if org_link:
+        content += get_action_button(org_link, "View Organization", PRIMARY_COLOR)
+
+    return get_base_html(content, f"Invitation accepted: {org_name}", f"{member_name} has joined {org_name}")
+