Fix/issues 260 264

[DNight-King]

Summary

This PR adds the remaining API surface and contract updates requested for the asset/event endpoints and snapshot caching behavior.

Changes

• Added GET /events to expose recent contract events from the indexer snapshot.
• Kept the event endpoint aligned with the existing snapshot-based API and OpenAPI docs.
• Documented conditional request support via If-None-Match / ETag behavior for snapshot-backed endpoints.
• Exposed indexed_at_ledger consistently across asset responses and documentation.
• Added sparse fieldset support via ?fields= on the asset endpoints.

Testing

• Validated the Rust API test suite locally (cargo test --quiet).

Related issues

closes #260
closes #261
closes #263
closes #264

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