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/App.jsx

@@ -6,6 +6,7 @@ import { DashboardPage } from "./pages/DashboardPage";
 import { TargetsPage } from "./pages/TargetsPage";
 import { TargetDetailPage } from "./pages/TargetDetailPage";
 import { QueryInsightsPage } from "./pages/QueryInsightsPage";
+import { AlertsPage } from "./pages/AlertsPage";
 import { AdminUsersPage } from "./pages/AdminUsersPage";
 
 function Protected({ children }) {
@@ -51,6 +52,14 @@ function Layout({ children }) {
             </span>
             <span className="nav-label">Query Insights</span>
           </NavLink>
+          <NavLink to="/alerts" className={navClass}>
+            <span className="nav-icon" aria-hidden="true">
+              <svg viewBox="0 0 24 24">
+                <path d="M15 17h5l-1.4-1.4A2 2 0 0 1 18 14.2V10a6 6 0 0 0-12 0v4.2a2 2 0 0 1-.6 1.4L4 17h5m6 0a3 3 0 0 1-6 0" />
+              </svg>
+            </span>
+            <span className="nav-label">Alerts</span>
+          </NavLink>
           {me?.role === "admin" && (
             <NavLink to="/admin/users" className={navClass}>
               <span className="nav-icon" aria-hidden="true">
@@ -99,6 +108,7 @@ export function App() {
                 <Route path="/targets" element={<TargetsPage />} />
                 <Route path="/targets/:id" element={<TargetDetailPage />} />
                 <Route path="/query-insights" element={<QueryInsightsPage />} />
+                <Route path="/alerts" element={<AlertsPage />} />
                 <Route path="/admin/users" element={<AdminUsersPage />} />
               </Routes>
             </Layout>