[David-Isaac-design]

Summary

Adds regression coverage for timestamped yield snapshot entries.

Closes #954

Changes

• Added assertions verifying that yield_snapshots.created_at is populated when daily yield snapshots are created.
• The test captures timestamps immediately before and after YieldService::create_daily_snapshots().
• Verifies that the stored snapshot timestamp falls within that execution window.
• Existing assertions continue to verify the user, earning balance, APY, and accrued interest.

No production yield logic was changed because the existing snapshot service already performs the database insertion within a transaction.

Validation

• git diff --check passes.
• Focused Rust test was attempted but could not be run because Cargo is unavailable in the local environment.
• Only backend/tests/yield_api_tests.rs was changed.

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[ONEONUORA]

Great job @David-Isaac-design