feat: Add get_post_metrics to all publishers for analytics

- Add abstract get_post_metrics() method to BasePublisher
- Implement get_post_metrics() in XPublisher using Twitter API v2
  - Returns: likes, comments, shares, retweets, quotes, impressions
- Implement get_post_metrics() in ThreadsPublisher using Meta Graph API
- Implement get_post_metrics() in FacebookPublisher with insights
- Implement get_post_metrics() in InstagramPublisher with insights

This enables the fetch_post_metrics task to collect engagement data
from all platforms, populating the analytics dashboard.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/publishers/base.py

@@ -68,6 +68,17 @@ class BasePublisher(ABC):
         """Eliminar un post."""
         pass
 
+    @abstractmethod
+    async def get_post_metrics(self, post_id: str) -> Optional[Dict]:
+        """
+        Obtener métricas de un post.
+
+        Returns:
+            Dict con métricas: likes, comments, shares, impressions, etc.
+            None si no se pueden obtener
+        """
+        pass
+
     def validate_content(self, content: str) -> bool:
         """Validar que el contenido cumple con los límites de la plataforma."""
         # Implementar en subclases según límites específicos