fix subscription unread count calculation

app/models.py

@@ -255,8 +255,11 @@ class Subscriptions(Model):
         q = """SELECT COUNT(*) AS unread_count
             FROM posts
             LEFT JOIN subscriptions ON subscriptions.thread_id = posts.thread_id
-            WHERE subscriptions.user_id = ? AND posts.created_at > subscriptions.last_seen"""
-        return db.fetch_one(q, self.user_id)['unread_count']
+            WHERE subscriptions.user_id = ? AND posts.created_at > subscriptions.last_seen AND posts.thread_id = ?"""
+        res = db.fetch_one(q, self.user_id, self.thread_id)
+        if res:
+            return res['unread_count']
+        return None
 
 class APIRateLimits(Model):
     table = 'api_rate_limits'