feat(pos): add gunicorn, marketplace B2B, and subscription billing (#7, #8, #12)

- Gunicorn production server with auto-scaled workers, run.sh, updated systemd service
- Marketplace B2B: cross-tenant inventory search, ordering, seller management with full UI
- Subscription billing: plan limits enforced on products/employees/branches, billing API + upgrade flow

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

docs/INSTALACION.md

@@ -105,11 +105,20 @@ print('Migraciones aplicadas correctamente')
 
 ### 8. Iniciar el POS
 
+**Desarrollo:**
 ```bash
 cd /home/Autopartes/pos
 python3 app.py
 ```
 
+**Produccion (Gunicorn):**
+```bash
+pip install gunicorn --break-system-packages
+sudo mkdir -p /var/log/nexus-pos
+cd /home/Autopartes/pos
+./run.sh
+```
+
 Acceder desde el navegador:
 
 ```
@@ -162,9 +171,10 @@ After=postgresql.service
 Wants=postgresql.service
 
 [Service]
-Type=simple
+Type=notify
 WorkingDirectory=/home/Autopartes/pos
-ExecStart=/usr/bin/python3 app.py
+ExecStartPre=/bin/mkdir -p /var/log/nexus-pos
+ExecStart=/usr/local/bin/gunicorn -c gunicorn.conf.py "app:create_app()"
 Restart=always
 RestartSec=5
 User=root