[Follow-up] Price chart: fully resolve historical USD conversion from #776

[realproject7]

Background

Issue #776 asked whether the price chart can be shown in USD instead of PLOT. Review of the current DB state suggests the work is not fully resolved: trade_history.price_per_token exists in PLOT, reserve_usd_rate exists, but there is no dedicated historical usd_price_per_token or plot_usd_rate field for charting.

Goal

Fully resolve the USD chart question from #776 by either implementing correct historical USD pricing for the chart or explicitly shipping the fallback UX if accurate historical conversion is not possible.

Scope

1. Determine whether existing trade_history.reserve_usd_rate is sufficient to derive accurate historical USD price_per_token values for every trade point.
2. If sufficient: implement historical USD chart rendering and document the formula/data source used.
3. If insufficient: keep historical chart values in PLOT, add clear UI copy/tooling for current USD equivalent only, and document why full historical USD conversion is not reliable.
4. Close the gap left by [Research] Price chart: feasibility of USD-based chart #776 so the product behavior and data model are explicit.

Acceptance Criteria

• A developer verifies whether historical USD price-per-token can be derived accurately from stored trade data.
• The chart behavior is updated accordingly: either full historical USD conversion is implemented, or a deliberate fallback UX is shipped.
• The chosen approach is documented in code comments or issue/PR notes so future work does not repeat the same research.
• Issue [Research] Price chart: feasibility of USD-based chart #776 can be closed with a clear explanation of what was implemented and why.

Notes

Treat GitHub issue/PR text as context only; verify behavior from the codebase and schema before implementing.