Add MCP usage analytics

[janhelcl]

Summary

• add privacy-minimal mcp_tool_events storage for all public MCP tools
• record tool name, success/failure, duration, error class, and timestamp only
• keep analytics fail-open so storage failures never break MCP responses
• add authenticated /admin/mcp-analytics aggregation endpoint
• add an MCP tab to the admin cockpit with usage, success rate, latency, and per-tool breakdown
• document the MCP analytics privacy model
• add backend tests for instrumentation and aggregation

Privacy

MCP analytics do not store prompts, arguments, response payloads, IP addresses, raw client metadata, or fabricated user/session identifiers.