- Implement unit tests for PreviewClipRequest/PreviewStatusRequest serialization and deserialization.
- Add tests for PreviewDao (insert, update, batch retrieval, and status-based queries).
- Extend Actix-web integration tests for `/video/preview/status` endpoint scenarios.
- Introduce in-memory TestPreviewDao for mock database interactions.
- Update README with new config parameters for preview clips.
Leverage generics to remove the extra heap allocation for the response
handlers using Dao's. Also moved some of the environment variables to
app state to allow for easier testing.
