feat: add integration tests for telemetry package

[jayfalls]

Summary

End-to-end tests for the telemetry package.

Files

• backend/shared/telemetry/integration_test.go

Tests

• Full trace: HTTP → NATS → Consumer → Database
• Metrics endpoint scrape
• Logger JSON output
• Usage event round-trip

Acceptance Criteria

• All integration tests pass
• NATS and PostgreSQL containers available

Design Docs

Read design/units/observability/ before starting:

• fsd.md
• architecture.md
• implementation.md

PR: (will be created)