feat: add CountPostsOfSite query and DB method

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

sql/queries/posts.sql

@@ -1,3 +1,12 @@
+-- name: CountPostsOfSite :one
+SELECT COUNT(*) FROM posts
+WHERE site_id = sqlc.arg(site_id) AND (
+    CASE CAST (sqlc.arg(post_filter) AS TEXT)
+        WHEN 'deleted' THEN deleted_at > 0
+        ELSE deleted_at = 0
+    END
+);
+
 -- name: SelectPostsOfSite :many
 SELECT *
 FROM posts