Add token metadata caching

[Miracle656]

Background

Every time Wraith processes a transfer event, it may need to resolve the token's symbol and decimals from the SAC contract via Soroban RPC. On high-volume ledgers, this adds significant latency and RPC load. Caching token metadata after the first lookup would reduce RPC calls dramatically.

What to build

Add an in-memory (or Postgres-backed) cache for token metadata: symbol, decimals, and name. Populate on first encounter; serve from cache on subsequent events.

Key files

• src/indexer.ts — where token metadata is currently looked up
• src/tokenCache.ts — create this module
• prisma/schema.prisma — optionally add a TokenMetadata model for persistence

Suggested execution

git checkout -b feat/token-metadata-caching

1. Create src/tokenCache.ts with a Map<contractId, { symbol, decimals, name }>
2. On service startup, populate the cache from any TokenMetadata rows in Postgres
3. In the indexer, check cache before making an RPC call; write to cache + DB after first fetch
4. Add a GET /tokens endpoint listing all cached tokens

Example commit message:
feat(indexer): add token metadata caching to reduce Soroban RPC calls

Acceptance criteria

• Second encounter of the same token does not make an RPC call
• Cache survives service restart (persisted to DB)
• GET /tokens returns all cached token metadata

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

[K1NGD4VID]

@K1NGD4VID has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

Hi!
Please, assign me this issue. I’ve worked on similar caching layers for metadata before, so I can implement an efficient in-memory + persistent cache, integrate it into the indexer, and expose the tokens endpoint with proper tests. ETA: 4–6 hours

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Congratulations, @K1NGD4VID! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on April 29, 2026.

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This issue has been marked as completed by a Drips Wave moderator for @K1NGD4VID as part of the Stellar Wave Program's 4th Wave 🥳

Moderator's reason: [Auto-assessed] The PR has been open for over 24 hours without maintainer objections and thoroughly addresses all requirements of the issue. It implements a multi-layer cache (in-memory + Prisma/Postgres), adds the required API endpoint, and includes comprehensive unit tests for both the cache logic and the XDR decoder.

😎 @K1NGD4VID: You earned 150 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here.

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ℹ️ Note: This issue was marked complete via moderation. Points were issued even though the GitHub issue remains open.