fix: pass editorial brief to LLM prompt + improve missing API key error

- 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>

backend/app/schemas.py

@@ -108,6 +108,7 @@ class GenerateContentRequest(BaseModel):
     platforms: List[str] = []              # new: multi-platform (overrides platform if non-empty)
     content_types: List[str] = []          # new: multi-type (overrides content_type if non-empty)
     topic_hint: Optional[str] = None
+    brief: Optional[str] = None           # editorial brief: technique + instructions for the LLM
     include_affiliates: bool = True
     provider: Optional[str] = None
     model: Optional[str] = None