Add email notification settings management

Implemented backend and frontend support for managing SMTP settings for email notifications. Includes API endpoints, database migration, and UI integration for configuring and testing email alerts.

frontend/src/styles.css

@@ -979,6 +979,74 @@ td {
   border-color: #7f1d1d;
 }
 
+.admin-settings-page h2 {
+  margin-top: 4px;
+  margin-bottom: 14px;
+}
+
+.admin-settings-page h3 {
+  margin-top: 0;
+  margin-bottom: 10px;
+}
+
+.admin-user-form {
+  gap: 10px;
+}
+
+.admin-users-table-wrap table tbody .admin-user-row td {
+  padding-top: 11px;
+  padding-bottom: 11px;
+  vertical-align: middle;
+}
+
+.user-col-id {
+  width: 64px;
+  color: #a8c0df;
+}
+
+.user-col-email {
+  font-weight: 600;
+}
+
+.role-pill {
+  display: inline-flex;
+  align-items: center;
+  padding: 4px 9px;
+  border-radius: 999px;
+  border: 1px solid transparent;
+  font-size: 11px;
+  font-weight: 700;
+  text-transform: uppercase;
+  letter-spacing: 0.05em;
+}
+
+.role-pill.role-admin {
+  color: #e9dcff;
+  border-color: #8f77e3;
+  background: #31235d;
+}
+
+.role-pill.role-operator {
+  color: #c7e7ff;
+  border-color: #4e8dc8;
+  background: #18375a;
+}
+
+.role-pill.role-viewer {
+  color: #cde2f8;
+  border-color: #4a6692;
+  background: #192b49;
+}
+
+.admin-smtp-form {
+  margin-top: 8px;
+  gap: 10px;
+}
+
+.admin-test-recipient {
+  min-width: 260px;
+}
+
 .muted {
   color: #9eb8d6;
 }