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/threads_publisher.py

@@ -225,3 +225,31 @@ class ThreadsPublisher(BasePublisher):
         """Eliminar un post de Threads (no soportado actualmente)."""
         # La API de Threads no soporta eliminación actualmente
         return False
+
+    async def get_post_metrics(self, post_id: str) -> Optional[Dict]:
+        """Obtener métricas de un post de Threads."""
+        if not self.access_token:
+            return None
+
+        try:
+            async with httpx.AsyncClient() as client:
+                url = f"{self.base_url}/{post_id}"
+                params = {
+                    "fields": "id,likes,replies,quotes,reposts",
+                    "access_token": self.access_token
+                }
+
+                response = await client.get(url, params=params)
+                response.raise_for_status()
+                data = response.json()
+
+                return {
+                    "likes": data.get("likes", 0),
+                    "comments": data.get("replies", 0),
+                    "shares": data.get("reposts", 0),
+                    "quotes": data.get("quotes", 0),
+                    "impressions": 0,  # No disponible en API pública
+                }
+
+        except httpx.HTTPError:
+            return None