feat(manager): add Nexus Instance Manager for demo orchestration

- Complete Flask-based control panel for multi-tenant POS instances
- Dashboard with global stats, system health, and recent demos
- Demo provisioning in 1 click with auto-expiration tracking
- Tenant management: activate/deactivate, reset data, delete
- Health monitoring: PostgreSQL, Redis, disk, memory, systemd services
- Migration orchestration UI for running schema updates across all tenants
- JWT authentication with manager_users table
- Dark theme SPA frontend with real-time search and actions
- systemd service file included

manager/blueprints/demos_bp.py

@@ -0,0 +1,42 @@
+"""Demo provisioning blueprint."""
+from flask import Blueprint, request, jsonify
+from blueprints.auth_bp import require_manager_auth
+from services import tenant_service
+
+demos_bp = Blueprint("demos", __name__, url_prefix="/api/demos")
+
+
+@demos_bp.route("", methods=["POST"])
+@require_manager_auth
+def create_demo():
+    data = request.get_json() or {}
+    name = data.get("name", "").strip()
+    email = data.get("email", "").strip()
+    days = data.get("days")
+    subdomain = data.get("subdomain", "").strip() or None
+    pin = data.get("pin", "0000").strip()
+
+    if not name:
+        return jsonify({"error": "Business name is required"}), 400
+
+    try:
+        result = tenant_service.create_demo(
+            name=name,
+            email=email,
+            demo_days=days,
+            subdomain=subdomain,
+            pin=pin
+        )
+        return jsonify({"data": result}), 201
+    except ValueError as e:
+        return jsonify({"error": str(e)}), 409
+    except Exception as e:
+        return jsonify({"error": str(e)}), 500
+
+
+@demos_bp.route("", methods=["GET"])
+@require_manager_auth
+def list_demos():
+    all_tenants = tenant_service.list_tenants(include_stats=True)
+    demos = [t for t in all_tenants if t.get("is_demo")]
+    return jsonify({"data": demos})