fix(analytics): expose flat-rate estimates on cost dashboards

[xiaoyaner0201]

Summary

Fixes #11459.

Claude Code (claude / cc) is correctly classified as a flat-rate subscription, so the analytics API reports $0 by default. The Costs and Analytics dashboards are estimate-oriented views, but neither originally had a token-price-equivalent view of subscription consumption.

This change preserves both meanings:

• ordinary analytics callers keep billed-cost semantics ($0 for flat-rate subscriptions);
• /dashboard/costs and /dashboard/analytics explicitly request token-price-equivalent estimates;
• the response reports whether estimates were included, so callers cannot mistake them for vendor billing records.

The defect is confirmed on the latest release/v3.8.50 line. This PR targets the active release/v3.8.51 branch because #11439 freezes v3.8.50.

Changes

• Add the exact opt-in query contract includeFlatRateEstimates=true; omitted, false, and unknown values retain the existing $0 behavior.
• Make both cost visualization pages request estimate mode and expose includesFlatRateEstimates in the response.
• Apply the selected mode consistently to summary, daily trend, model/provider/account/API-key breakdowns, service-tier comparison, and preset summaries.
• Keep rollups estimate-capable after raw-history retention and group cost inputs at day/provider/model/service-tier granularity so non-linear pricing clamps reconcile across summary and breakdowns.
• Add regression coverage for default and estimate modes, archived usage, rollup persistence, aggregation consistency, and both dashboard wiring paths.

Non-goals

• No change to budgets, quota enforcement, routing, stored credentials, or provider billing semantics.
• Estimates are not presented as actual subscription invoices or vendor billing data.
• This PR does not address the separate analytics-page latency issue.

Testing

• ✅ RED wiring test failed before the Analytics overview opt-in was added.
• ✅ node --import tsx/esm --test tests/integration/integration-wiring.test.ts tests/unit/usage-analytics-route.test.ts tests/unit/flat-rate-cost-5552.test.ts tests/unit/usage-analytics.test.ts tests/unit/db-usageanalytics-split.test.ts tests/unit/db-usageanalytics-sources-split.test.ts — 148 tests, 144 passed / 4 skipped / 0 failed
• ✅ npm run typecheck:core
• ✅ npx prettier --check on the changed files
• ✅ npm run check:changelog-integrity
• ✅ npm run check:file-size
• ✅ commit hooks: ESLint autofix, docs sync, any-budget, tracked-artifacts
• ✅ git diff --check
• ✅ Isolated SQLite canary: estimate-mode summary and provider breakdown reconcile after day-level grouping

Review notes

The query parameter is deliberately opt-in rather than changing the analytics default. This keeps compatibility with #5552 / #10773 while restoring analytical usefulness on the two dashboards that label cost as an estimate.