Show current conversation cost in footer

[witbrock]

What changed

• add an authenticated, content-free aggregate for the selected conversation and the current actor/organisation since server restart
• reuse canonical provider-reported usage, pricing, and call-ID deduplication semantics
• overlay cumulative live-turn cost without double-counting during durable persistence handover
• show a responsive footer pill with estimated, partial, unavailable, and non-billable states plus accessible details
• invalidate and refetch the runtime total when server identity changes

User impact

The footer now shows the current conversation's running estimated model cost and the actor-scoped total since restart. Missing usage or pricing remains partial/unavailable rather than being displayed as zero. The detail panel explains coverage and that the value is an estimate, not a provider invoice.

Validation

• 22 focused backend tests passed after the final patch
• 293 focused frontend tests passed
• focused Ruff and frontend static lint passed
• JavaScript syntax and git diff --check passed
• authenticated isolated-server browser validation passed for desktop, 600 px responsive layout, details disclosure, and Escape dismissal