Add Amazon Bedrock Nova S2S Live Podcasting POC with guardrails

[mevana]

Adds a proof-of-concept demonstrating Amazon Nova Sonic bidirectional streaming for AI-powered live podcast generation. Two AI hosts (Matthew and Tiffany) engage in multi-turn spoken dialogue about AWS topics.

Features:

• Flask web app with SSE for real-time transcript streaming
• CLI modes: text, audio, and mixed interaction
• Topic validation (60+ AWS keywords)
• PII output filtering (emails, phones, account IDs, access keys)
• System and per-turn prompt guardrails
• Property-based and unit tests (33 tests)

Issue #, if available:

Description of changes:

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