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/queue.go

@@ -4,6 +4,7 @@ import (
 	"log"
 	"net/http"
 
+	"github.com/gin-gonic/gin"
 	"github.com/imc-vibe/backend/internal/mail"
 )
 
@@ -13,48 +14,33 @@ func NewQueueHandler() *QueueHandler {
 	return &QueueHandler{}
 }
 
-func (h *QueueHandler) List(w http.ResponseWriter, r *http.Request) {
-	if r.Method != http.MethodGet {
-		Error(w, http.StatusMethodNotAllowed, "method not allowed")
-		return
-	}
-
+func (h *QueueHandler) List(c *gin.Context) {
 	entries, err := mail.GetQueue()
 	if err != nil {
 		log.Printf("Queue error: %v", err)
-		Error(w, http.StatusServiceUnavailable, "failed to get queue: "+err.Error())
+		Error(c, http.StatusServiceUnavailable, "failed to get queue: "+err.Error())
 		return
 	}
 
-	Success(w, entries)
+	Success(c, entries)
 }
 
-func (h *QueueHandler) Requeue(w http.ResponseWriter, r *http.Request) {
-	if r.Method != http.MethodPost {
-		Error(w, http.StatusMethodNotAllowed, "method not allowed")
-		return
-	}
-
-	id := extractID(r.URL.Path)
+func (h *QueueHandler) Requeue(c *gin.Context) {
+	id := c.Param("id")
 	if err := mail.RequeueMail(id); err != nil {
-		Error(w, http.StatusInternalServerError, "failed to requeue mail")
+		Error(c, http.StatusInternalServerError, "failed to requeue mail")
 		return
 	}
 
-	Success(w, map[string]string{"message": "mail requeued"})
+	Success(c, map[string]string{"message": "mail requeued"})
 }
 
-func (h *QueueHandler) Delete(w http.ResponseWriter, r *http.Request) {
-	if r.Method != http.MethodDelete {
-		Error(w, http.StatusMethodNotAllowed, "method not allowed")
-		return
-	}
-
-	id := extractID(r.URL.Path)
+func (h *QueueHandler) Delete(c *gin.Context) {
+	id := c.Param("id")
 	if err := mail.DeleteFromQueue(id); err != nil {
-		Error(w, http.StatusInternalServerError, "failed to delete from queue")
+		Error(c, http.StatusInternalServerError, "failed to delete from queue")
 		return
 	}
 
-	NoContent(w)
+	NoContent(c)
 }