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/health_bp.py

@@ -0,0 +1,18 @@
+"""Health check blueprint."""
+from flask import Blueprint, jsonify
+from blueprints.auth_bp import require_manager_auth
+from services import health_service
+
+health_bp = Blueprint("health", __name__, url_prefix="/api/health")
+
+
+@health_bp.route("", methods=["GET"])
+@require_manager_auth
+def full_health():
+    return jsonify(health_service.get_full_health_report())
+
+
+@health_bp.route("/tenant/<db_name>", methods=["GET"])
+@require_manager_auth
+def tenant_health(db_name):
+    return jsonify(health_service.get_tenant_health(db_name))