feat: add user_management plugin with session management & API key support

[kgobakis]

Plugin: User Management

Multi-user authentication, chat isolation, and token usage tracking with PostgreSQL.

Features

• PostgreSQL Database — Store users, sessions, and token usage
• Multi-user Auth — Replace single login with per-user accounts
• Chat Isolation — Tag each context with user_id, filter in UI
• Token Tracker — Hook into LLM calls, log input/output tokens per user
• Admin Panel — WebUI modal to view usage stats + export
• Session Manager — Track active sessions per user with expiry
• API Key Store — Generate & store per-user API keys in PostgreSQL for automation
• API Auth Middleware — Validate incoming API key, resolve to user_id
• POST /api/chat Endpoint — Accept external messages via API key for automations
• Rate Limiter — Throttle API requests per key to prevent abuse

Files

All plugin files under plugins/user_management/