Harden backend against critical and high security vulnerabilities

- Fix WebSocket CheckOrigin to use proper url.Parse instead of string stripping
- Add admin auth checks to Users/User GraphQL queries
- Remove GraphQL GET transport to prevent CSRF via cross-site links
- Add application-level IP-based login rate limiting (5 attempts/min)
- Add path traversal bounds check on radio file upload
- Require DEV_MODE for GraphQL introspection and playground
- Move notes backend endpoint behind admin middleware
- Add dedicated Nginx rate limit zone for GraphQL (10r/s)

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

backend/handlers/handle_auth.go

@@ -226,6 +226,11 @@ func (store *Store) RefreshToken(ctx *gin.Context) {
 }
 
 func (store *Store) Login(ctx *gin.Context) {
+	if !store.LoginLimiter.Allow(ctx.ClientIP()) {
+		ctx.JSON(http.StatusTooManyRequests, gin.H{"error": "too many login attempts, please try again later"})
+		return
+	}
+
 	var input UserCredentials
 	if err := ctx.ShouldBindBodyWithJSON(&input); err != nil {
 		ctx.JSON(http.StatusBadRequest, gin.H{"error": "invalid request body"})