Get current odds for a fixture.
Lookup mode: required fixtureId + optional bookmakers, optional filtering flags.
If since is provided, returns odds updates with changedAt >= since.
Your API key (query-based auth for v5).
Single fixture ID (e.g. id1400003160574219).
Comma/space-separated bookmaker slugs (default: all).
changedAt >= since (ms)
If set, filter by marketActive flag.
If set, filter by mainLine flag.
Returns FixtureOddsResponse (fixture meta + odds + bookmaker meta)