Improve docker-compose with readiness and healthchecks

[walterosc]

This PR improves the robustness of the Kafka Connect service in Docker Compose:

• Adds explicit depends_on conditions with service_healthy for broker and schema registry.
• Adds a healthcheck to Kafka Connect to ensure its REST API is ready.
• Ensures connectors are only registered after Kafka Connect is up and reachable.
• Avoids premature connector registration which previously caused data not to be ingested in QuestDB.

These changes help avoid race conditions and improve system stability during startup.

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