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/services/content.py

@@ -15,6 +15,7 @@ def generate_post_text(
     llm_provider: LLMProvider,
     platform: str,
     topic_hint: str | None = None,
+    brief: str | None = None,
 ) -> str:
     """Generate social media post text based on a character profile.
 
@@ -23,6 +24,7 @@ def generate_post_text(
         topic_hint: Optional topic suggestion to guide generation.
         llm_provider: LLM provider instance for text generation.
         platform: Target platform (e.g. 'instagram', 'facebook', 'tiktok', 'youtube').
+        brief: Optional editorial brief with narrative technique and detailed instructions.
 
     Returns:
         Generated post text as a string.
@@ -78,8 +80,17 @@ def generate_post_text(
     if topic_hint:
         topic_instruction = f" The post should be about: {topic_hint}."
 
+    # Brief is the highest-priority instruction — it overrides defaults
+    brief_instruction = ""
+    if brief:
+        brief_instruction = (
+            f"\n\nISRUZIONI OBBLIGATORIE DAL BRIEF EDITORIALE:\n{brief}\n"
+            f"Rispetta TUTTE le istruzioni del brief. "
+            f"Il brief ha priorità su qualsiasi altra indicazione."
+        )
+
     prompt = (
-        f"{guidance}{topic_instruction}\n\n"
+        f"{guidance}{topic_instruction}{brief_instruction}\n\n"
         f"Write the post now. Output ONLY the post text, nothing else."
     )