Add alert management functionality in backend and frontend

This commit introduces alert management capabilities, including creating, updating, listing, and removing custom SQL-based alerts in the backend. It adds the necessary database migrations, API endpoints, and frontend pages to manage alerts, enabling users to define thresholds and monitor system health effectively.

frontend/src/styles.css

@@ -225,6 +225,7 @@ a {
 
 input,
 select,
+textarea,
 button {
   background: #0f2750;
   color: var(--text);
@@ -730,6 +731,11 @@ details[open] .collapse-chevron {
   border-color: #be3f63;
 }
 
+.small-btn {
+  min-height: 30px;
+  padding: 4px 10px;
+}
+
 button {
   cursor: pointer;
 }
@@ -763,6 +769,121 @@ td {
   border-color: #7f1d1d;
 }
 
+.muted {
+  color: #9eb8d6;
+}
+
+.alerts-subtitle {
+  margin-top: 2px;
+  color: #a6c0df;
+}
+
+.alerts-kpis .alerts-kpi {
+  display: grid;
+  gap: 2px;
+}
+
+.alerts-kpi strong {
+  font-size: 32px;
+  line-height: 1;
+}
+
+.alerts-kpi.warning {
+  border-color: #9a6426;
+  background: linear-gradient(180deg, #342713, #251b0f);
+}
+
+.alerts-kpi.alert {
+  border-color: #a53a46;
+  background: linear-gradient(180deg, #381520, #2b1018);
+}
+
+.alerts-list {
+  display: grid;
+  gap: 10px;
+  max-height: 520px;
+  overflow: auto;
+  padding-right: 3px;
+}
+
+.alert-item {
+  border-radius: 12px;
+  border: 1px solid #375d8f;
+  background: #10294f;
+  padding: 12px;
+}
+
+.alert-item.warning {
+  border-color: #8a6d34;
+  background: linear-gradient(180deg, #2f2516, #261f15);
+}
+
+.alert-item.alert {
+  border-color: #9f3e4a;
+  background: linear-gradient(180deg, #361822, #2d131b);
+}
+
+.alert-item-head {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  margin-bottom: 6px;
+}
+
+.alert-item-head small {
+  color: #c3d5ef;
+  margin-left: auto;
+}
+
+.alert-item p {
+  margin: 4px 0;
+  color: #d3e5fb;
+}
+
+.alert-message {
+  font-size: 13px;
+  color: #b6cae8;
+}
+
+.alert-badge {
+  display: inline-block;
+  border-radius: 999px;
+  padding: 3px 8px;
+  font-size: 11px;
+  font-weight: 700;
+  border: 1px solid transparent;
+}
+
+.alert-badge.warning {
+  color: #f9d8a8;
+  background: #3a2c16;
+  border-color: #a1742f;
+}
+
+.alert-badge.alert {
+  color: #fecaca;
+  background: #3a1620;
+  border-color: #ad4552;
+}
+
+.alert-form .field-full {
+  grid-column: 1 / -1;
+}
+
+.alert-form textarea {
+  width: 100%;
+  min-height: 90px;
+  resize: vertical;
+  font-family: "JetBrains Mono", "Consolas", monospace;
+  font-size: 13px;
+}
+
+.alert-form-actions {
+  display: flex;
+  justify-content: flex-end;
+  gap: 8px;
+}
+
 .range-picker {
   display: flex;
   gap: 8px;