feat: add pagination controls to admin post list

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

models/paging.go

@@ -26,3 +26,12 @@ func (p PageInfo) PreviousPage() int {
 func (p PageInfo) NextPage() int {
 	return p.CurrentPage + 1
 }
+
+// Pages returns a slice of page numbers for rendering numbered pagination.
+func (p PageInfo) Pages() []int {
+	pages := make([]int, p.TotalPages)
+	for i := range pages {
+		pages[i] = i + 1
+	}
+	return pages
+}