Add support for pg_stat_statements configuration in Targets

This commit introduces a `use_pg_stat_statements` flag for targets, allowing users to enable or disable the use of `pg_stat_statements` for query insights. It includes database schema changes, backend logic, and UI updates to manage this setting in both creation and editing workflows.

frontend/src/pages/QueryInsightsPage.jsx

@@ -75,8 +75,10 @@ export function QueryInsightsPage() {
     (async () => {
       try {
         const t = await apiFetch("/targets", {}, tokens, refresh);
-        setTargets(t);
-        if (t.length > 0) setTargetId(String(t[0].id));
+        const supported = t.filter((item) => item.use_pg_stat_statements !== false);
+        setTargets(supported);
+        if (supported.length > 0) setTargetId(String(supported[0].id));
+        else setTargetId("");
       } catch (e) {
         setError(String(e.message || e));
       } finally {
@@ -133,11 +135,17 @@ export function QueryInsightsPage() {
     <div className="query-insights-page">
       <h2>Query Insights</h2>
       <p>Note: This section requires the <code>pg_stat_statements</code> extension on the monitored target.</p>
+      {targets.length === 0 && !loading && (
+        <div className="card">
+          No targets with enabled <code>pg_stat_statements</code> are available.
+          Enable it in <strong>Targets Management</strong> for a target to use Query Insights.
+        </div>
+      )}
       {error && <div className="card error">{error}</div>}
       <div className="card query-toolbar">
         <div className="field">
           <label>Target</label>
-          <select value={targetId} onChange={(e) => setTargetId(e.target.value)}>
+          <select value={targetId} onChange={(e) => setTargetId(e.target.value)} disabled={!targets.length}>
             {targets.map((t) => (
               <option key={t.id} value={t.id}>
                 {t.name}