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/scheduler.go

@@ -33,7 +33,7 @@ func StartScheduler(ctx context.Context, pool *pgxpool.Pool) {
 			c, cancel := context.WithTimeout(context.Background(), 3*time.Minute)
 			defer cancel()
 			if err := s.AggregateSlowQueries(c, 30); err != nil {
-				log.Printf("pgmonitor: slow queries: %v", err)
+				log.Printf("pgmonitor: slow queries snapshot: %v", err)
 			}
 			if err := s.EstimateTableBloat(c); err != nil {
 				log.Printf("pgmonitor: bloat: %v", err)