Bound watched transaction set growth

[hman38705]

Issue 018: Bound watched transaction set growth

Area: Backend
Priority: Medium
Files: services/api/src/blockchain.rs

Problem

watched_txs can grow without TTL or cap for never-finalized transaction hashes. Memory grows unbounded over time.

Acceptance Criteria

• Add TTL and max-size policy for watched hashes\n- Evictions are observable via metrics and logs\n- Memory growth test is added\n- Eviction policy is documented

[codefather2026]

@codefather2026 has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

Hi maintainer, i will like to solve this issue

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[observerr411]

@observerr411 has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

I would like to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @observerr411 to this issue.

[drips-wave]

Congratulations, @observerr411! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on April 29, 2026.

🧑‍💻 @observerr411: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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[drips-wave]

This issue has been completed by @observerr411 as part of the Stellar Wave Program's 4th Wave 🥳

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