Add target owners and alert notification management.

This commit implements the addition of `target_owners` and `alert_notification_events` tables, enabling management of responsible users for targets. Backend and frontend components are updated to allow viewing, assigning, and notifying target owners about critical alerts via email.

frontend/src/styles.css

@@ -749,6 +749,36 @@ button {
   gap: 10px;
 }
 
+.owner-grid {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 8px;
+}
+
+.owner-chip {
+  display: inline-flex;
+  align-items: center;
+  gap: 6px;
+  padding: 6px 10px;
+  border-radius: 10px;
+  border: 1px solid #315a8d;
+  background: #10284d;
+  color: #d9e8fb;
+  font-size: 12px;
+  font-weight: 600;
+}
+
+.owner-chip.active {
+  border-color: #52c7f8;
+  background: #174377;
+}
+
+.owner-chip input {
+  margin: 0;
+  width: 14px;
+  height: 14px;
+}
+
 .primary-btn {
   font-weight: 650;
   letter-spacing: 0.01em;
@@ -1621,6 +1651,25 @@ select:-webkit-autofill {
   margin: 8px 0 10px 16px;
 }
 
+.owner-row {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  margin: -6px 0 12px;
+  flex-wrap: wrap;
+}
+
+.owner-pill {
+  display: inline-flex;
+  align-items: center;
+  padding: 4px 10px;
+  border-radius: 999px;
+  border: 1px solid #3f7abc;
+  background: #17355f;
+  color: #d9ebff;
+  font-size: 12px;
+}
+
 .chart-tooltip {
   background: #0f1934ee;
   border: 1px solid #2f4a8b;