- Install nodemailer + @types/nodemailer (needed by Plan 03)
- Create healthProbeService.ts with 4 probers: document_ai, llm_api, supabase, firebase_auth
- Each probe makes a real authenticated API call
- Each probe returns structured ProbeResult with status, latency_ms, error_message
- LLM probe uses cheapest model (claude-haiku-4-5) with max_tokens 5
- Supabase probe uses getPostgresPool().query('SELECT 1') not PostgREST
- Firebase Auth probe distinguishes expected vs unexpected errors
- runAllProbes orchestrator uses Promise.allSettled for fault isolation
- Results persisted via HealthCheckModel.create() after each probe
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