Add Session runtime metrics tab

[sam2tom]

Summary

• add a Metrics tab beside Conversation and Trace for the selected Session
• reuse the Dashboard RuntimeTrendPanel and four CPU, memory, token, and uptime cards
• keep PostgreSQL durable history backend-neutral; uptime remains explicitly Live-only in History mode

Validation

• Web TypeScript check
• 45 focused Dashboard/runtime tests
• Web production build
• independent blind review: no blockers

Scope

Frontend only. Based directly on merged PR #30 main; does not restore ClickHouse code or configuration.

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