feat: GraphQL endpoint alongside the REST surface

[satyakwok]

Currently the indexer exposes Etherscan-API-compatible REST. Add a GraphQL endpoint for clients that benefit from precise field selection (frontend dashboards, analytics tools).

Why

REST forces over-fetch (full Block JSON when caller only needs txCount). GraphQL clients can request just the fields they need — important for explorer + scan-style UIs that paginate large lists.

Scope

• Schema for: block, transaction, address, tokenTransfer, log
• Wire on top of existing Postgres reads
• Single endpoint at /graphql
• Persisted-queries support optional but useful for production frontends

Out of scope

• Subscriptions over WebSocket — separate issue, requires PG NOTIFY plumbing
• Federation / schema stitching with other Sentriscloud services

Acceptance

• /graphql endpoint live
• Schema doc + examples in repo
• Performance: 95th-percentile query latency under 200ms on the standard Postgres