Add tests for top-movers computation edge cases: new assets, zero-supply

[christabel888]

Context: Edge cases the ranking computation needs to not crash or misrank on.

Where:

• tests/ -- new test file or extend an existing analytics test file

What to do:

• Test a newly-created asset (no 24h-ago baseline to diff against -- shouldn't divide by zero or show a nonsensical infinite percent gain).
• Test a zero-supply/zero-holder asset.

Suggested approach:

1. Write a test seeding a brand-new asset (created today, no 24h-ago snapshot) through Add /api/v1/rankings/top-movers endpoint: 24h percent change by holders/volume #33's ranking computation and assert it doesn't crash, divide by zero, or show an infinite/NaN percent change.
2. Write a second test for a zero-supply or zero-holder asset and assert the same.
3. Decide explicitly what these edge cases should display (e.g. "new" badge instead of a percentage) and assert on that specific output, not just "doesn't crash."

Watch out for:

• These are exactly the inputs a public, gameable endpoint attracts (someone could create a zero-supply asset specifically to see what breaks) -- treat this as a correctness issue, not just a nice-to-have test.

Definition of done:

• Both edge cases have explicit test coverage and don't panic or return garbage values