feat: Phase B learning + hashtag profiles Pro-only lock

- Approve action saves post as reference example in character's content_rules
- Keep last 5 approved examples per character (auto-rotating)
- Inject last 3 approved examples as few-shot in LLM system prompt
- Lock YouTube/TikTok hashtag profile tabs for Freemium users (Pro only)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/routers/content.py

@@ -315,12 +315,28 @@ def approve_post(
     db: Session = Depends(get_db),
     current_user: User = Depends(get_current_user),
 ):
-    """Approve a post (set status to 'approved')."""
+    """Approve a post (set status to 'approved'). Also saves it as a reference example for the character."""
     post = db.query(Post).filter(Post.id == post_id, Post.user_id == current_user.id).first()
     if not post:
         raise HTTPException(status_code=404, detail="Post not found")
     post.status = "approved"
     post.updated_at = datetime.utcnow()
+
+    # Save as reference example for future few-shot learning (keep last 5 per character)
+    if post.text_content and post.character_id:
+        character = db.query(Character).filter(Character.id == post.character_id).first()
+        if character:
+            examples = character.content_rules or {}
+            approved_examples = examples.get("approved_examples", [])
+            approved_examples.append({
+                "platform": post.platform_hint or "general",
+                "text": post.text_content[:500],  # truncate for prompt efficiency
+            })
+            # Keep only last 5
+            examples["approved_examples"] = approved_examples[-5:]
+            character.content_rules = examples
+            character.updated_at = datetime.utcnow()
+
     db.commit()
     db.refresh(post)
     return post