Implement update_metadata for bounty titles and add backend PATCH end…

[elcabasa]

Description

fixes #231
This PR resolves issue #231 by implementing the update_metadata functionality, allowing maintainers to correct bounty titles on non-finalized bounties without having to cancel and recreate them. The implementation spans both the Soroban smart contract and the off-chain backend indexing API endpoint.

Scope of Changes

Smart Contract (contract.rs)

• Added the BountyMetadataUpdated contract type struct for indexer event streams.
• Implemented pub fn update_metadata(env: Env, bounty_id: u64, maintainer: Address, new_title: String) with the following security guardrails:
  • Enforces caller authorization via maintainer.require_auth().
  • Rejects execution with a MaintainerMismatch error if a non-owner attempts an modification.
  • Rejects execution if the target bounty status evaluates to Released, Refunded, or Expired.
  • Safely overwrites the existing state on success and publishes the BountyMetadataUpdated event containing both the old and new titles.

Backend REST API

• Created a PATCH /api/bounties/:id/metadata controller routing endpoint.
• Introduced off-chain validation guards matching the contract logic (preventing updates on unauthorized access or finalized/expired statuses).

Acceptance Criteria Verified

• Only the original maintainer can call update_metadata.
• Rejected on Released, Refunded, or Expired statuses.
• Emits BountyMetadataUpdated event with old and new title.
• Added backend /api/bounties/:id/metadata PATCH endpoint.
• Added unit tests checking success, missing authentication, and status constraints.

Testing Matrix

• Executed smart contract unit testing assertions covering edge cases:

  cargo test

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