fix: add Next.js API route handler to proxy with correct host headers

backend/internal/api/middleware/tenant.go

@@ -2,6 +2,7 @@ package middleware
 
 import (
 	"context"
+	"log"
 	"net/http"
 	"strings"
 
@@ -17,7 +18,16 @@ const SubdomainKey tenantContextKey = "subdomain"
 func TenantDetector(tenantRepo *repository.TenantRepository) func(http.Handler) http.Handler {
 	return func(next http.Handler) http.Handler {
 		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-			host := r.Host
+			// 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
+			}
+			
+			log.Printf("TenantDetector: host = %s (from headers), path = %s", host, r.RequestURI)
 			
 			// Extract subdomain
 			// Examples:
@@ -39,6 +49,8 @@ func TenantDetector(tenantRepo *repository.TenantRepository) func(http.Handler)
 				}
 			}
 			
+			log.Printf("TenantDetector: extracted subdomain = %s", subdomain)
+			
 			// Add subdomain to context
 			ctx := context.WithValue(r.Context(), SubdomainKey, subdomain)
 			
@@ -46,7 +58,10 @@ func TenantDetector(tenantRepo *repository.TenantRepository) func(http.Handler)
 			if subdomain != "" && subdomain != "dash" && subdomain != "api" && subdomain != "localhost" {
 				tenant, err := tenantRepo.FindBySubdomain(subdomain)
 				if err == nil && tenant != nil {
+					log.Printf("TenantDetector: found tenant %s for subdomain %s", tenant.ID.String(), subdomain)
 					ctx = context.WithValue(ctx, TenantIDKey, tenant.ID.String())
+				} else {
+					log.Printf("TenantDetector: tenant not found for subdomain %s (err=%v)", subdomain, err)
 				}
 			}