feat(db): enhance PostgreSQL statistics monitoring and error handling

Updated the PostgreSQL monitoring service to improve handling of `pg_stat_statements` availability. Introduced a new method to check if the extension is queryable and updated the response structure to include availability status and hints. Enhanced the documentation to clarify the requirements for enabling `pg_stat_statements`. Adjusted related components to reflect these changes, ensuring better user feedback in the monitoring interface.

web/src/lib/components/monitoring/MonitoringPostgresTab.svelte

@@ -281,12 +281,18 @@
 					<CardTitle>Медленные запросы</CardTitle>
 					<CardDescription>
 						Источник: {queries?.source ?? '—'}
-						{#if overview && !overview.pg_stat_statements_enabled}
-							· pg_stat_statements не включён
+						{#if queries?.statements_available === false || (overview && !overview.pg_stat_statements_enabled)}
+							· pg_stat_statements недоступен
 						{/if}
 					</CardDescription>
 				</CardHeader>
 				<CardContent>
+					{#if queries?.statements_hint}
+						<Alert class="mb-4">
+							<AlertTitle>Нет статистики запросов</AlertTitle>
+							<AlertDescription>{queries.statements_hint}</AlertDescription>
+						</Alert>
+					{/if}
 					<Table>
 						<TableHeader>
 							<TableRow>

web/src/lib/monitoring/postgres.ts

@@ -48,6 +48,8 @@ export type PostgresQueriesResponse = {
 	collected_at: string;
 	source: string;
 	items: PostgresQueryRow[];
+	statements_available?: boolean;
+	statements_hint?: string;
 };
 
 export type PostgresLockRow = {