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/facebook_publisher.py

@@ -250,3 +250,46 @@ class FacebookPublisher(BasePublisher):
 
         except httpx.HTTPError:
             return False
+
+    async def get_post_metrics(self, post_id: str) -> Optional[Dict]:
+        """Obtener métricas de un post de Facebook."""
+        if not self.access_token:
+            return None
+
+        try:
+            async with httpx.AsyncClient() as client:
+                url = f"{self.base_url}/{post_id}"
+                params = {
+                    "fields": "likes.summary(true),comments.summary(true),shares,insights.metric(post_impressions,post_engaged_users)",
+                    "access_token": self.access_token
+                }
+
+                response = await client.get(url, params=params)
+                response.raise_for_status()
+                data = response.json()
+
+                # Extraer métricas
+                likes = data.get("likes", {}).get("summary", {}).get("total_count", 0)
+                comments = data.get("comments", {}).get("summary", {}).get("total_count", 0)
+                shares = data.get("shares", {}).get("count", 0)
+
+                # Insights (pueden no estar disponibles)
+                impressions = 0
+                reach = 0
+                insights = data.get("insights", {}).get("data", [])
+                for insight in insights:
+                    if insight.get("name") == "post_impressions":
+                        impressions = insight.get("values", [{}])[0].get("value", 0)
+                    elif insight.get("name") == "post_engaged_users":
+                        reach = insight.get("values", [{}])[0].get("value", 0)
+
+                return {
+                    "likes": likes,
+                    "comments": comments,
+                    "shares": shares,
+                    "impressions": impressions,
+                    "reach": reach,
+                }
+
+        except httpx.HTTPError:
+            return None