feat: add chapkit_simple_multistep_model compose overlay

[mortenoh]

Summary

• Adds compose.multistep.yml, a docker compose overlay for the chap-models/chapkit_simple_multistep_model chapkit service, mirroring the existing compose.ewars.yml pattern.
• Image is pulled from ghcr.io/chap-models/chapkit_simple_multistep_model:latest (public, multi-arch — no platform: pin needed).
• Uses host port 5003:8000 (EWARS holds 5002) and the same urllib healthcheck probe that EWARS ships with.

How I tested

Brought the stack up with docker compose -f compose.yml -f compose.multistep.yml up -d against a local DHIS2 and walked through the modeling app end-to-end:

• Service self-registered to chap-core's /v2/services registry, template synced with healthStatus=live, usesChapkit=true.
• _sync_chapkit_configured_models pulled the defaulted additional_continuous_covariates (rainfall, mean_temperature, mean_relative_humidity) from the chapkit service onto the configured-model row.
• Prediction via the modeling app: Success in 8 s.
• Evaluation via the modeling app: dry run Valid import: 18/18, full import Success in 33 s.

Notes

• Target Makefile wiring (make restart / make chap-version) still references only the EWARS overlay. Intentionally leaving it alone in this PR — happy to follow up if we want the default dev stack to include this service too.
• DHIS2's bundled Lao PDR dataset has no province-level mean_relative_humidity data item, so the default configured model (which includes humidity) can't pass _validate_full_dataset. Tested against a sibling configured model with additional_continuous_covariates=["rainfall","mean_temperature"] to bypass that. No code change needed in chap-core — just a data shape the bundled sample doesn't cover.

Test plan

• docker compose -f compose.yml -f compose.multistep.yml config parses cleanly
• multistep container reaches (healthy) status
• /v2/services lists chapkit-simple-multistep-model
• Prediction through the modeling app completes end-to-end
• Evaluation through the modeling app completes end-to-end