docs: complete setup guide for DeepSeek provider

[artcava]

The file docs/setup-deepseek.md has been added as a draft in docs/readme-architecture-overhaul.

Tasks:

• Validate all config keys against local.settings.json.example
• Verify model names and API endpoint (https://api.deepseek.com/v1)
• Confirm compatibility with Microsoft.Extensions.AI.OpenAI adapter (OpenAI-compatible API)
• Add note about context window limits for long RSS articles
• Cross-check troubleshooting against real production errors
• Review and merge as part of docs/readme-architecture-overhaul PR

Related: docs/setup-falai.md (companion), docs/configuration.md, Architecture: HybridAiService

[github-actions]

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