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.

internal/pgmonitor/snapshot.go

@@ -66,11 +66,15 @@ func (s *Service) RefreshMetricsSnapshot(ctx context.Context) error {
 
 // AggregateSlowQueries stores top statements snapshot.
 func (s *Service) AggregateSlowQueries(ctx context.Context, limit int) error {
-	if !s.statementsEnabled(ctx) {
+	if !s.statementsQueryable(ctx) {
 		return s.UpsertSnapshot(ctx, "slow_queries", []QueryStat{})
 	}
 	items, err := queryTopStatements(ctx, s.pool, clampLimit(limit, 20, 100))
 	if err != nil {
+		if isPgStatStatementsUnavailable(err) {
+			s.markStatementsUnavailable()
+			return s.UpsertSnapshot(ctx, "slow_queries", []QueryStat{})
+		}
 		return err
 	}
 	return s.UpsertSnapshot(ctx, "slow_queries", items)