- Add 'brief' field to GenerateContentRequest schema
- Pass brief from router to generate_post_text service
- Inject brief as mandatory instructions in LLM prompt with highest priority
- Return structured error when LLM provider/API key not configured
- Show dedicated warning banner with link to Settings when API key missing
Fixes: content ignoring editorial brief, unhelpful API key error messages
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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