Refactor EO API surface around ingestions, artifacts, collections, and OGC coverage

[abyot]

Summary

This PR refactors the current publication slice into a clearer native API flow:

ingestions -> artifacts -> Zarr/NetCDF storage -> collections -> /ogcapi coverage

Changes

• rename public downloads API to ingestions
• split native FastAPI/OpenAPI groups into Ingestions, Artifacts, and Collections
• move native collection routes into their own module
• keep /artifacts and /collections as top-level public resources
• keep publication as part of ingestion via publish=true
• make Zarr the explicit preferred managed artifact format, with NetCDF fallback
• add a public guide at docs/ingestion_artifact_api_guide.md

Result

The branch now presents a more coherent API for:

• ingesting remote EO data
• materializing managed local artifacts as Zarr when possible
• exposing native artifact and collection state
• publishing gridded data through pygeoapi and OGC API Coverages