Fix price history query amplification

[NateIsern]

Motivation

• The bounded history sampler previously escalated one request into hundreds of concurrent indexed findOne lookups, which can saturate a small MongoDB connection pool and cause availability issues on the public /api/price/history endpoint.
• The expensive path was reachable for normal windows (default 7d) once the sampled series exceeded HISTORY_MAX_POINTS, making the amplification the common case rather than an edge case.
• The fix must eliminate massive DB fanout while preserving the capped, evenly-thinned history returned to clients.

Description

• Replace the over-threshold per-target findOne fanout with a single sorted PricePoint.find({ timestamp: { $gte: cutoff } }) query and in-memory processing to thin the series.
• Implement an efficient linear pass that walks the retrieved docs and selects nearest documents for each evenly spaced target time, then ensures the newest point is retained; this preserves the HISTORY_MAX_POINTS-bounded, oldest→newest output.
• Change localized to server/routes/price.ts in the fetchBoundedPriceHistory helper, removing the Promise.all multi-query approach and keeping external behavior and response shape unchanged.

Testing

• Ran npm run typecheck, which completed successfully.
• Ran the test suite with npm test (Vitest), and all tests passed (4 files, 42 tests).
• Attempted npm test -- --runInBand which failed because Vitest does not accept Jest-style --runInBand options (this is a test runner option mismatch, not a functional regression).

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Codex Task