API cost monitoring + alerting per user (automatic thresholds)

[mikececco]

Roadmap ref: 3.1 Ottimizzazioni

Deadline: December 23, 2026 | Priority: Critical
Note: Roadmap says 'implement as soon as Layer 1 is live — don't wait for Stage 3'

Description

Per-user API cost monitoring with automatic threshold alerts. Critical for maintaining the €0.25/user/month target.

Acceptance Criteria

• Per-user cost tracking (tokens + API calls + external services)
• Automatic alerts at configurable thresholds (80%, 90%, 100% of budget)
• Admin dashboard: top cost users, cost distribution, trend lines
• Daily/weekly cost summary emails (opt-in for admins)
• Anomaly detection: flag sudden cost spikes
• Foundation for budget enforcement (issue Budget enforcement — graduate from Phase 0 observe-only #14)