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/service_test.go

@@ -1,6 +1,11 @@
 package pgmonitor
 
-import "testing"
+import (
+	"errors"
+	"testing"
+
+	"github.com/jackc/pgx/v5/pgconn"
+)
 
 func TestClampLimit(t *testing.T) {
 	if clampLimit(0, 20, 100) != 20 {
@@ -31,3 +36,13 @@ func TestNewServiceNilPool(t *testing.T) {
 		t.Fatal("expected nil service")
 	}
 }
+
+func TestIsPgStatStatementsUnavailable(t *testing.T) {
+	err := &pgconn.PgError{Code: "55000", Message: "pg_stat_statements must be loaded via shared_preload_libraries"}
+	if !isPgStatStatementsUnavailable(err) {
+		t.Fatal("55000")
+	}
+	if isPgStatStatementsUnavailable(errors.New("other")) {
+		t.Fatal("unrelated")
+	}
+}