Add GraphQL subscriptions for live prices

[Miracle656]

Background

Mercurius already runs over the existing Fastify server; subscriptions are a small lift compared to building a separate WebSocket stack. Subscribing clients gets parity with the existing REST /ws channel via the familiar GraphQL idiom.

What to build

Add a priceUpdated(pair: String!) GraphQL subscription that emits whenever the ingester produces a new price.

Key files

• lens/src/api/graphql.ts — register the subscription
• lens/src/ingester/ — publish on new prices via Mercurius pubsub
• New: README example showing graphql-ws client usage

Suggested execution

git checkout -b feat/graphql-subscriptions

1. Enable Mercurius subscriptions (subscription: true in options)
2. Add a priceUpdated(pair) subscription resolver backed by app.graphql.pubsub
3. Ingester publishes { pair, price, ts } after every new price
4. Filter by pair in the resolver
5. Write a small graphql-ws example client

Example commit message:
feat(graphql): add priceUpdated subscription via Mercurius

Acceptance criteria

• Subscribing to priceUpdated(pair: "XLM/USDC") streams updates
• Unsubscribing closes the channel cleanly
• README documents the client snippet

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Drips Wave · Complexity: Medium · 150 points
Comment below to request assignment. PR must include Closes #[this issue].

[mikkyvans0-source]

@mikkyvans0-source has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

hello am a full-stack blockchain dev willing to work on this project

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