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/admin_bp.py

@@ -0,0 +1,35 @@
+"""Admin dashboard blueprint."""
+from flask import Blueprint, jsonify
+from blueprints.auth_bp import require_manager_auth
+from services import tenant_service, migration_service
+
+admin_bp = Blueprint("admin", __name__, url_prefix="/api/admin")
+
+
+@admin_bp.route("/stats", methods=["GET"])
+@require_manager_auth
+def dashboard_stats():
+    return jsonify(tenant_service.get_dashboard_stats())
+
+
+@admin_bp.route("/migrations", methods=["GET"])
+@require_manager_auth
+def list_migrations():
+    return jsonify({
+        "migrations": migration_service.list_available_migrations(),
+        "tenants": migration_service.get_tenant_versions()
+    })
+
+
+@admin_bp.route("/migrations/run-all", methods=["POST"])
+@require_manager_auth
+def run_all_migrations():
+    result = migration_service.run_all_pending_migrations()
+    return jsonify(result)
+
+
+@admin_bp.route("/migrations/run/<version>", methods=["POST"])
+@require_manager_auth
+def run_specific_migration(version):
+    result = migration_service.run_migration_on_all_tenants(version)
+    return jsonify({"results": result})