Cross-tenant object-store write in SOCFortress CoPilot upload endpoint

SOCFortress CoPilot focuses on providing a single pane of glass for all your security operations needs. Prior to 0.1.76, `POST /api/agent_data_store/upload` in `backend/app/data_store/data_store_routes.py` permits `customer_user` callers to choose `bucket_name` and `object_name` without an allowlist, tenant prefix, ownership check or content restriction. The only file check requires a nonempty filename. The operations in `backend/app/data_store/data_store_operations.py` create missing buckets and upsert objects, allowing existing objects belonging to any tenant to be overwritten. Targets include `sysmon-configs/<customer_code>/sysmon_config.xml` distributed to managed Windows endpoints, `copilot-case-report-templates/<name>` rendered server-side, and case evidence or forensic artifacts under `copilot-cases/<case_id>/<file>` and `velociraptor-artifacts/<agent_id>/<flow_id>/<file>`. A lowest-privilege user can tamper with cross-tenant evidence, blind endpoint telemetry, poison report templates for later code execution, plant data or exhaust storage. Sibling list, download and delete routes remain restricted to admin/analyst; the exposed operation is the upload write path. This issue is fixed in version 0.1.76.