Implement deterministic ETag caching for GraphQL responses

[Fatimasanusi]

Description

Repeatedly querying the identical state of a fractional vault wastes database I/O and increases client wait times.

Architecture & Context

Aligning with GRAPHQL_IMPLEMENTATION.md, the backend must implement deterministic ETag generation. By hashing the current ledger sequence and the requested Vault ID, the Fastify server can return 304 Not Modified for unchanged data, leveraging browser-level caching.

Technical Requirements

• Generate ETags using crypto.createHash('sha256') based on ledger state.
• Intercept GraphQL requests to validate If-None-Match headers.
• Bypass Prisma queries entirely if the ETag validates.

Acceptance Criteria

• Unchanged vault queries return 304 Not Modified.
• ETag generation adds less than 5ms overhead.
• Database query volume drops by 40% under repeated polling.