audit: cost telemetry retention + re-identification policy

**Summary:** v0.7.3 emits per-generation `cost` and `tokens_input_total` to Azure App Insights. Joined with `project_id` + `session_id`, the per-session economic activity of a specific customer is reconstructible.

**Recommended audit:**
1. Document retention TTL on the `generation` event type — confirm it matches what customers were told in TOS.
2. Evaluate whether `cost` should be bucketed/quantized at emit time (e.g., to the nearest cent) or kept exact for billing reconciliation.
3. Verify the `(project_id, session_id, cost)` tuple is not exposed to anyone outside billing.

**Origin:** Deferred from v0.7.3 release review (Compliance, P2).