[haocheng23]

Summary

Adds three read-only insight panels to the Overview page, all driven by the same time window as the traffic chart:

• Credit-expiry distribution — credits grouped by the calendar day they lapse, one colour per account, plus a soonest-expiry (FEFO) table showing the daily burn rate needed to spend each batch before it expires.
• Usage breakdown — token split (prompt / completion / total), prefix-cache hit rate, error count and latency; switchable between account / model / realm dimensions and sortable by total tokens, requests, errors or latency.
• Credit deduction history — sums upstream usage.credit per account and per model, with KPI tiles for credits deducted, matched tokens, average credits per 1M tokens, valid samples and cache-hit rate. Requests without a credit reading are excluded; nothing is back-filled; model rates come from the regional price catalog.

Backend:

• New GET /api/logs/usage endpoint (internal/console/logs.go).
• SummarizeRequestUsage aggregation in internal/store/request_logs.go (single grouped query + credit sums), exposed through RequestRecorder.Usage with a 10s TTL cache that mirrors the existing stats cache.
• New RequestUsage types in internal/accounts/request_logs.go.

Frontend:

• fetchRequestUsage + RequestUsage types in frontend/src/api/logs.ts.
• New ExpiryDistribution.tsx and UsagePanels.tsx components; wired into OverviewPage.tsx reusing statsQuery so the panels follow the page time window (including the new Today window).
• Adds a fixed chart identity palette (--chart-1..8, light/dark) so charts can distinguish 4+ accounts without abusing the semantic status colours (--success / --warning / --danger).

Relationship to #336

This is complementary to #336 (feat(console): add token usage and quota page). #336 adds a standalone usage/quota page; this PR embeds breakdown panels directly in the Overview page and adds the shared /api/logs/usage aggregation. Both touch internal/store/request_logs.go, internal/logs/recorder.go and frontend/src/i18n/messages.ts, so whichever lands second will need a small rebase, but the features are additive and independent.

Test plan

• go test ./...
• go vet ./...
• cd worker && npm test (untouched by this PR)
• cd frontend && npm run build && npm run lint (build green, lint 0 errors)
• Added changelog/unreleased/credit-expiry-distribution.md (EN + 中文)
• No tokens, auth blobs, raw captures, or host details in the diff
• Manual: open Overview, switch between Today / 1h / 24h / 7d / 30d / custom and confirm all three panels refresh in lockstep with the traffic chart.