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/routers/content.py

@@ -78,9 +78,21 @@ def generate_content(
     model = request.model or _get_setting(db, "llm_model", current_user.id)
 
     if not provider_name:
-        raise HTTPException(status_code=400, detail="LLM provider not configured. Set 'llm_provider' in settings.")
+        raise HTTPException(
+            status_code=400,
+            detail={
+                "message": "Provider AI non configurato. Vai in Impostazioni → Provider AI per scegliere il provider e inserire la API key.",
+                "missing_settings": True,
+            },
+        )
     if not api_key:
-        raise HTTPException(status_code=400, detail="LLM API key not configured. Set 'llm_api_key' in settings.")
+        raise HTTPException(
+            status_code=400,
+            detail={
+                "message": "API key non configurata. Vai in Impostazioni → Provider AI per inserire la tua API key.",
+                "missing_settings": True,
+            },
+        )
 
     # Build character dict for content service
     char_dict = {
@@ -98,6 +110,7 @@ def generate_content(
         llm_provider=llm,
         platform=request.effective_platform,
         topic_hint=request.topic_hint,
+        brief=request.brief,
     )
 
     # Generate hashtags