v1.4: Segurança multi-tenant, file serving via API e UX humanizada

-  Validação cross-tenant no login e rotas protegidas
-  File serving via /api/files/{bucket}/{path} (eliminação DNS)
-  Mensagens de erro humanizadas inline (sem pop-ups)
-  Middleware tenant detection via headers customizados
-  Upload de logos retorna URLs via API
-  README atualizado com changelog v1.4 completo

backend/internal/api/middleware/tenant.go

@@ -16,15 +16,25 @@ func TenantDetector(tenantRepo *repository.TenantRepository) func(http.Handler)
 	return func(next http.Handler) http.Handler {
 		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 			// Get host from X-Forwarded-Host header (set by Next.js proxy) or Host header
-			host := r.Header.Get("X-Forwarded-Host")
-			if host == "" {
-				host = r.Header.Get("X-Original-Host")
-			}
-			if host == "" {
-				host = r.Host
-			}
+			// Priority order: X-Tenant-Subdomain (set by Next.js middleware) > X-Forwarded-Host > X-Original-Host > Host
+			tenantSubdomain := r.Header.Get("X-Tenant-Subdomain")
 			
-			log.Printf("TenantDetector: host = %s (from headers), path = %s", host, r.RequestURI)
+			var host string
+			if tenantSubdomain != "" {
+				// Use direct subdomain from Next.js middleware
+				host = tenantSubdomain
+				log.Printf("TenantDetector: using X-Tenant-Subdomain = %s", tenantSubdomain)
+			} else {
+				// Fallback to extracting from host headers
+				host = r.Header.Get("X-Forwarded-Host")
+				if host == "" {
+					host = r.Header.Get("X-Original-Host")
+				}
+				if host == "" {
+					host = r.Host
+				}
+				log.Printf("TenantDetector: host = %s (from headers), path = %s", host, r.RequestURI)
+			}
 			
 			// Extract subdomain
 			// Examples:
@@ -33,17 +43,28 @@ func TenantDetector(tenantRepo *repository.TenantRepository) func(http.Handler)
 			// - dash.localhost -> dash (master admin)
 			// - localhost -> (institutional site)
 			
-			parts := strings.Split(host, ".")
 			var subdomain string
 			
-			if len(parts) >= 2 {
-				// Has subdomain
-				subdomain = parts[0]
-				
+			// If we got the subdomain directly from X-Tenant-Subdomain, use it
+			if tenantSubdomain != "" {
+				subdomain = tenantSubdomain
 				// Remove port if present
 				if strings.Contains(subdomain, ":") {
 					subdomain = strings.Split(subdomain, ":")[0]
 				}
+			} else {
+				// Extract from host
+				parts := strings.Split(host, ".")
+				
+				if len(parts) >= 2 {
+					// Has subdomain
+					subdomain = parts[0]
+					
+					// Remove port if present
+					if strings.Contains(subdomain, ":") {
+						subdomain = strings.Split(subdomain, ":")[0]
+					}
+				}
 			}
 			
 			log.Printf("TenantDetector: extracted subdomain = %s", subdomain)