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/blueprints/inventory_bp.py

@@ -204,6 +204,23 @@ def create_item():
     if not branch_id:
         return jsonify({'error': 'branch_id required'}), 400
 
+    # Plan limit check
+    from services.billing import check_limit, next_plan, PLANS, get_plan
+    conn = get_tenant_conn(g.tenant_id)
+    cur_count = conn.cursor()
+    cur_count.execute("SELECT count(*) FROM inventory WHERE is_active = true")
+    current_products = cur_count.fetchone()[0]
+    cur_count.close()
+
+    allowed, limit, current = check_limit(g.tenant_id, 'max_products', current_products)
+    if not allowed:
+        conn.close()
+        plan_key = get_plan(g.tenant_id)
+        nxt = next_plan(plan_key)
+        nxt_name = PLANS[nxt]['name'] if nxt else 'Enterprise'
+        return jsonify({'error': f'Plan limit reached ({limit} products). Upgrade to {nxt_name}.'}), 403
+
+    conn.close()
     conn = get_tenant_conn(g.tenant_id)
     cur = conn.cursor()