- Add source and confidence fields to RuntimeUsageWindow, ModelUsageEntry,
TopSession, RuntimeUsageCurrent, and ProviderUsageScrapeResult schemas
- _build_window() assigns source based on data origin:
provider_native > provider_api_rate_limit > local_jsonl_estimate
- _build_current() tags token_limit_source and cost_limit_source
- Frontend relabels 'Current session'/'All models' to 'API rate limit'
- Shows source label and confidence in usage strip
- Changes 'did not return active usage windows' to 'did not return
API rate-limit windows for percent + reset diagnostics'
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