feat: subdomain routing por tenant — refac-xxx.nexusautoparts.com

- Nginx wildcard config: *.nexusautoparts.com routes to POS app with X-Tenant-Subdomain header
- middleware_tenant.py: resolves subdomain -> tenant_id via nexus_master.tenants.subdomain
- auth_bp: login and employee list endpoints accept tenant from subdomain, URL param, or body
- login.html: auto-detects tenant from subdomain, shows business name, falls back to ?tenant=ID
- tenant_manager: generates subdomain slug from business name on provision_tenant()
- Migration v1.2: adds subdomain column + unique index to tenants table
- setup-nginx.sh: one-command install script for the nginx config

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

nginx/setup-nginx.sh

@@ -0,0 +1,24 @@
+#!/bin/bash
+# Setup Nginx config for Nexus POS subdomain routing
+# Usage: sudo bash setup-nginx.sh
+
+set -euo pipefail
+
+CONF_SRC="/home/Autopartes/nginx/nexus-pos.conf"
+CONF_DEST="/etc/nginx/sites-available/nexus-pos"
+
+echo "==> Copying nginx config..."
+sudo cp "$CONF_SRC" "$CONF_DEST"
+
+echo "==> Creating symlink in sites-enabled..."
+sudo ln -sf "$CONF_DEST" /etc/nginx/sites-enabled/
+
+echo "==> Testing nginx config..."
+if sudo nginx -t; then
+    echo "==> Config OK. Reloading nginx..."
+    sudo systemctl reload nginx
+    echo "==> Done. Subdomain routing active."
+else
+    echo "!!! Nginx config test failed. NOT reloading."
+    exit 1
+fi