Telemetry coverage gaps

[ZIJ]

Summary

• adds a working review of telemetry coverage across sessions-api, api-edge, the dashboard, and the TypeScript SDK
• separates pipeline robustness from product journey coverage gaps
• proposes the first implementation slice for mixed dashboard/API/SDK correlation and user-journey reconstruction

Key points

• keep sessions-api telemetry event names free-form; do not add a central taxonomy gate
• add shared correlation headers across dashboard, REST, SDK, edge proxying, and sessions-api
• add a first-party browser telemetry collector so UI failures land in the ClickHouse flight recorder
• instrument edge auth/signup/billing/model-billing paths, which currently own activation-critical steps

Testing

• git diff --cached --check
• ASCII scan for the new doc