Add display history REST API

[suppenkasper0815]

Summary

• add a new REST endpoint for display history values
• expose the stored diagram datapoints through the JSON API

Review notes

• Race conditions: addressed by guarding the diagram state with a mutex for both background sampling and API/rendering access.
• Thread safety: the shared graph buffer is now read under the same lock as the writer tasks.
• Const correctness: getters now return snapshots instead of exposing mutable state directly.
• Memory usage: the change keeps the existing fixed-size buffer and adds only a small mutex and a copied snapshot for the API path.
• JSON size: the payload contains only the current values array plus metadata, with no extra per-entry overhead.
• OOM risk: no dynamic allocations were introduced in the new request path; the response uses the existing fixed-size array.
• AsyncWebServer conformity: the handler follows the existing AsyncWebServer pattern and uses the shared JSON response helper.
• Code style: the implementation matches the surrounding project conventions.

Verification

• Full build: pio run ✅