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>

pos/templates/dashboard.html

@@ -1378,6 +1378,18 @@
           Reportes
         </a>
 
+        <div class="sidebar__section-label">Gestión</div>
+
+        <a href="/pos/marketplace" class="nav-link">
+          <span class="nav-link__icon">
+            <svg width="16" height="16" viewBox="0 0 16 16" fill="none">
+              <path d="M1 3h14v2l-1.5 1.5V14H2.5V6.5L1 5V3z" stroke="currentColor" stroke-width="1.4" stroke-linejoin="round"/>
+              <path d="M6 10h4v4H6z" fill="currentColor" opacity="0.4"/>
+            </svg>
+          </span>
+          Marketplace B2B
+        </a>
+
         <div class="sidebar__section-label">Sistema</div>
 
         <a href="/pos/config" class="nav-link">
@@ -1673,6 +1685,7 @@
   </div><!-- end app-shell -->
 
 
+  <script src="/pos/static/js/i18n.js"></script>
   <script src="/pos/static/js/app-init.js"></script>
   <script src="/pos/static/js/sidebar.js"></script>
   <script src="/pos/static/js/dashboard.js"></script>