feat(backend): update Square integration, validators, and image validation

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/internal/validators/validators.go

@@ -1,10 +1,12 @@
 package validators
 
 import (
+	"fmt"
 	"github.com/go-playground/validator/v10"
 	"reflect"
 	"regexp"
 	"strings"
+	"time"
 )
 
 var Validate *validator.Validate
@@ -31,3 +33,16 @@ func IsValidID(id string) bool {
 	}
 	return validIDRegex.MatchString(id)
 }
+
+// ParseCursor splits a "createdAt|id" cursor string into its components.
+func ParseCursor(cursor string) (time.Time, string, error) {
+	parts := strings.SplitN(cursor, "|", 2)
+	if len(parts) != 2 {
+		return time.Time{}, "", fmt.Errorf("invalid cursor format")
+	}
+	t, err := time.Parse(time.RFC3339, parts[0])
+	if err != nil {
+		return time.Time{}, "", fmt.Errorf("invalid cursor created_at: %w", err)
+	}
+	return t, parts[1], nil
+}