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/x_publisher.py

@@ -278,6 +278,36 @@ class XPublisher(BasePublisher):
         except tweepy.TweepyException:
             return False
 
+    async def get_post_metrics(self, post_id: str) -> Optional[Dict]:
+        """Obtener métricas de un tweet."""
+        if not self.client:
+            return None
+
+        try:
+            # Obtener tweet con métricas públicas
+            tweet = self.client.get_tweet(
+                id=post_id,
+                tweet_fields=['public_metrics', 'created_at']
+            )
+
+            if not tweet.data:
+                return None
+
+            metrics = tweet.data.public_metrics or {}
+
+            return {
+                "likes": metrics.get("like_count", 0),
+                "comments": metrics.get("reply_count", 0),
+                "shares": metrics.get("retweet_count", 0),
+                "retweets": metrics.get("retweet_count", 0),
+                "quotes": metrics.get("quote_count", 0),
+                "impressions": metrics.get("impression_count", 0),
+                "bookmarks": metrics.get("bookmark_count", 0),
+            }
+
+        except tweepy.TweepyException:
+            return None
+
     async def get_followers(self, max_results: int = 100) -> List[Dict]:
         """Obtener lista de followers recientes."""
         if not self.client: