precompute_engine has no source abstraction — ingest is hardcoded to HTTP #300

The HTTP server is the only ingest path in precompute_engine, and it is hardcoded directly in engine.rs::run(). There is no trait or interface representing an ingest source.

The closest shared logic is route_decoded_samples() in ingest_handler.rs, which both HTTP handlers call. However, this function is private and coupled to the private IngestState struct, making it inaccessible to any alternative ingest path. SeriesRouter has a clean public interface (route_group_batch, broadcast_flush, broadcast_shutdown), but there is no abstraction above it that unifies how sources push data into the router.

As a result, adding any new ingest source (file, Kafka-native, etc.) requires modifying engine.rs::run() directly and duplicating routing logic, rather than implementing a defined interface. This makes the engine difficult to extend and tests each new source in isolation from the common routing path.