feat(backend): implement HTTP API, auth, chat, and admin surfaces

Expose learning, registration controls, mentor chat, realtime WebSocket, and staff administration.

backend/internal/api/pagination.go

@@ -0,0 +1,66 @@
+package api
+
+import (
+	"net/http"
+	"reflect"
+	"strconv"
+)
+
+const (
+	defaultPageSize = 10
+	maxPageSize     = 100
+)
+
+type pageParams struct {
+	Page     int
+	PageSize int
+	Offset   int
+}
+
+// parsePageParams reads page + pageSize (or limit) from the query string.
+// Defaults: page=1, pageSize=10. pageSize is capped at 100.
+func parsePageParams(r *http.Request) pageParams {
+	page := 1
+	pageSize := defaultPageSize
+
+	if v := r.URL.Query().Get("page"); v != "" {
+		if n, err := strconv.Atoi(v); err == nil && n > 0 {
+			page = n
+		}
+	}
+	if v := r.URL.Query().Get("pageSize"); v != "" {
+		if n, err := strconv.Atoi(v); err == nil && n > 0 {
+			pageSize = n
+		}
+	} else if v := r.URL.Query().Get("limit"); v != "" {
+		if n, err := strconv.Atoi(v); err == nil && n > 0 {
+			pageSize = n
+		}
+	}
+	if pageSize > maxPageSize {
+		pageSize = maxPageSize
+	}
+
+	return pageParams{
+		Page:     page,
+		PageSize: pageSize,
+		Offset:   (page - 1) * pageSize,
+	}
+}
+
+func writePage(w http.ResponseWriter, items any, total int64, p pageParams) {
+	if items == nil {
+		items = []any{}
+	} else {
+		v := reflect.ValueOf(items)
+		if v.Kind() == reflect.Slice && v.IsNil() {
+			items = reflect.MakeSlice(v.Type(), 0, 0).Interface()
+		}
+	}
+	writeJSON(w, http.StatusOK, map[string]any{
+		"items":    items,
+		"total":    total,
+		"page":     p.Page,
+		"pageSize": p.PageSize,
+	})
+}