Switch from net/http to gin-gonic web framework

- Added gin-gonic v1.10.0 dependency
- Refactored router.go: clean route groups with middleware chains
- Refactored all handlers to use gin.Context instead of http.ResponseWriter/*http.Request
- Simplified response helpers (JSON, Error, Success, Created, NoContent)
- Clean auth middleware using Gin's c.Set() for context
- Cleaner route definitions with path parameters (e.g., /domains/:name/users/:id)
- Admin routes moved to /api/admin group with RequireAdmin middleware

backend/internal/api/handlers/response.go

@@ -1,8 +1,7 @@
 package handlers
 
 import (
-	"encoding/json"
-	"net/http"
+	"github.com/gin-gonic/gin"
 )
 
 type Response struct {
@@ -16,26 +15,22 @@ type Meta struct {
 	Page  int `json:"page,omitempty"`
 }
 
-func JSON(w http.ResponseWriter, status int, data interface{}) {
-	w.Header().Set("Content-Type", "application/json")
-	w.WriteHeader(status)
-	json.NewEncoder(w).Encode(Response{Data: data})
+func JSON(c *gin.Context, status int, data interface{}) {
+	c.JSON(status, Response{Data: data})
 }
 
-func Error(w http.ResponseWriter, status int, message string) {
-	w.Header().Set("Content-Type", "application/json")
-	w.WriteHeader(status)
-	json.NewEncoder(w).Encode(Response{Error: message})
+func Error(c *gin.Context, status int, message string) {
+	c.JSON(status, Response{Error: message})
 }
 
-func Success(w http.ResponseWriter, data interface{}) {
-	JSON(w, http.StatusOK, data)
+func Success(c *gin.Context, data interface{}) {
+	JSON(c, 200, data)
 }
 
-func Created(w http.ResponseWriter, data interface{}) {
-	JSON(w, http.StatusCreated, data)
+func Created(c *gin.Context, data interface{}) {
+	JSON(c, 201, data)
 }
 
-func NoContent(w http.ResponseWriter) {
-	w.WriteHeader(http.StatusNoContent)
+func NoContent(c *gin.Context) {
+	c.Status(204)
 }