fix(backend): reduce Kafka startup delay

[betterclever]

This PR optimizes Kafka consumers to prevent blocking backend startup and reduces rebalance delays during restarts.

Changes:

• Implemented non-blocking Kafka connection (background init).
• Reduced Kafka sessionTimeout to 6s and heartbeatInterval to 2s to speed up group rebalancing.
• Enabled NestJS shutdown hooks in main.ts to ensure consumers leave their groups gracefully.
• Added explicit logging for Kafka disconnects.
• Fixed a typo in workflows.service.ts.

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