Refactor pagination system with flexible ordering support

This commit overhauls the pagination system to support flexible ordering
beyond the default created_at timestamp:

- Change CursorKey from fixed byte array to structured object with ID and Value
- Add OrderField and OrderDirection types to support different sorting options
- Update SQLFragment to dynamically generate SQL based on field and direction
- Modify cursor navigation logic to consider custom ordering
- Simplify Position type to use string constants instead of numeric values
- Update Page struct to include reference to Cursor

These changes allow paginated queries to be ordered by different fields
(created_at, updated_at, name) in either ascending or descending order
while maintaining consistent cursor-based pagination behavior.

Signed-off-by: gearnode <bryan@frimin.fr>

pkg/page/order_field.go

@@ -0,0 +1,71 @@
+// Copyright (c) 2025 Probo Inc <hello@getprobo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package page
+
+import (
+	"fmt"
+)
+
+type (
+	OrderField interface {
+		Column() string
+	}
+
+	BaseOrderField string
+)
+
+const (
+	OrderFieldCreatedAt BaseOrderField = "CREATED_AT"
+	OrderFieldUpdatedAt BaseOrderField = "UPDATED_AT"
+	OrderFieldName      BaseOrderField = "NAME"
+)
+
+func (of BaseOrderField) String() string {
+	return string(of)
+}
+
+func (of BaseOrderField) Column() string {
+	switch of {
+	case OrderFieldCreatedAt:
+		return "created_at"
+	case OrderFieldUpdatedAt:
+		return "updated_at"
+	case OrderFieldName:
+		return "name"
+	default:
+		return ""
+	}
+}
+
+func (of BaseOrderField) MarshalText() ([]byte, error) {
+	return []byte(of.String()), nil
+}
+
+func (of *BaseOrderField) UnmarshalText(data []byte) error {
+	val := string(data)
+
+	switch val {
+	case OrderFieldCreatedAt.String():
+		*of = OrderFieldCreatedAt
+	case OrderFieldUpdatedAt.String():
+		*of = OrderFieldUpdatedAt
+	case OrderFieldName.String():
+		*of = OrderFieldName
+	default:
+		return fmt.Errorf("invalid BaseOrderField value: %q", val)
+	}
+
+	return nil
+}