feat: add GraphQL subscriptions for live transfer and host-fn log streams

[collinsezedike]

Summary

• Adds a Subscription type over WebSocket at /graphql/subscriptions, streaming transferAdded and hostFnLogAdded events as they're ingested, each filterable by contractId
• Wires the indexer to emit host-fn log events alongside the existing transfer events so subscribers see both in real time
• Adds a bounded, drop-oldest async iterator plus a bufferedAmount check on send so a slow consumer can't grow server memory unboundedly

Implementation notes

Built directly on the graphql and ws packages already in package.json rather than adding graphql-ws, using a small graphql-ws-style JSON protocol (subscribe / next / complete / error) and attaching resolvers onto a buildSchema-built schema the same way makeExecutableSchema does internally.

Files ended up under src/graphql/ (alongside the existing src/graphql/server.ts) rather than the src/api/graphql.ts / src/api/subscriptions.ts paths mentioned in the issue, to match the repo's existing GraphQL code layout.

Test plan

• npx tsc --noEmit passes
• npm test passes (24 suites / 244 tests, including 6 new subscription tests covering subscribe/receive, contractId filtering on both channels, unsubscribe via complete, and rapid-fire backpressure)
• Manual verification against a live RPC-backed deployment (not done — no live Soroban RPC access in this environment)

Closes #99

[drips-wave]

@collinsezedike Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits