Implement a deterministic ETag hashing interceptor for caching immutable historical vault metrics

[Damola-Sodiq]

Description

Engineer a Fastify plugin that calculates deterministic SHA-1 hashes of historical JSON data to enable aggressive client-side ETag caching.

Architecture & Context

Historical vault performance metrics in tokenized-fractional never change once a ledger closes. However, clients continually re-fetch them. Generating deterministic ETags allows the server to return 304 Not Modified, saving immense bandwidth.

Technical Requirements

• Serialize outgoing JSON payloads using a strict, alphabetical key-sorting algorithm.
• Calculate and append the ETag header before flushing the response stream.
• Intercept If-None-Match headers to short-circuit the execution pipeline immediately.

Acceptance Criteria

• Unchanged historical data queries return 304 Not Modified.
• CPU overhead for hashing is mitigated by caching the ETag alongside the DB query.
• Frontend React Query caches correctly respect the 304 responses without redrawing.