fix(pos): eliminar redirect a nexusautoparts.com en subdomain middleware

Subdominio desconocido ya no redirige a dominio externo. En su lugar,
continua sin tenant y el login pide el tenant manualmente.
Agregados 'nexus', 'pos', 'app', 'dashboard' a subdominios reservados.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pos/middleware_tenant.py

@@ -15,7 +15,7 @@ from flask import request, g, redirect
 from tenant_db import get_master_conn
 
 # Domains that should NOT be treated as tenant subdomains
-_RESERVED = {'www', 'api', 'admin', 'mail', 'staging', 'dev'}
+_RESERVED = {'www', 'api', 'admin', 'mail', 'staging', 'dev', 'nexus', 'pos', 'app', 'dashboard'}
 
 # Cache: subdomain -> {tenant_id, name} (cleared on app restart)
 _subdomain_cache = {}
@@ -103,9 +103,8 @@ def resolve_tenant():
             g.tenant_subdomain = subdomain
             return
         else:
-            # Unknown subdomain: redirect to main site (only for page loads, not API)
-            if not request.path.startswith('/pos/api/'):
-                return redirect('https://nexusautoparts.com', code=302)
+            # Unknown subdomain: just continue without tenant (login will ask for it)
+            pass
 
     # Fallback: ?tenant=ID URL parameter
     tenant_param = request.args.get('tenant')