Full Investigation Async

Start an investigation asynchronously and return immediately.

This is the correct pattern for long-running investigations behind an ALB/CDN:

• Create a placeholder investigation row immediately
• Spawn the investigation in the background
• Caller polls GET /api/investigations/{id} (or MCP get_investigation) for results

Body format (FastAPI multi-body-params): { "request": {"query": "...", "system_message": "..."}, "mcps": ["sentry-mcp", ...] // optional — falls back to team DB config }