Add Kafka/Redpanda streaming backend support

[fuzziecoder]

Motivation

• Introduce an explicit, configurable streaming backend so the event layer can target Kafka-compatible systems (Apache Kafka or Redpanda) and surface which backend emitted each event.

Description

• Add EVENT_STREAM_BACKEND: Literal["kafka", "redpanda"] to Settings and keep existing bootstrap/topic settings in backend/config.py.
• Update the event publisher (backend/events/publisher.py) to include stream_backend in emitted event envelopes and to make log messages reflect the selected backend when initialization or dependency issues occur.
• Document the streaming choices and usage scenarios in backend/README.md and extend backend/.env.example with event-streaming keys and topic defaults.
• Add unit tests backend/tests/test_event_publisher.py to verify backend tagging in published events and noop behavior when streaming is disabled.

Testing

• Ran the focused test suite with PYTHONPATH=. pytest backend/tests/test_event_publisher.py backend/tests/test_event_streaming.py, and all tests passed (4 passed, 6 warnings).

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Codex Task

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Project	Deployment	Actions	Updated (UTC)
flexi-roaster	Ready	Preview, Comment	Feb 25, 2026 1:20pm

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch codex/implement-real-time-streaming-pipelines

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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