Add RLS policy to allow anyone to read published posts

Previously only authors and admins could read post_drafts rows,
so published posts were invisible to other users.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Yushin-L

supabase/migrations/007_published_posts_read_policy.sql

@@ -0,0 +1,7 @@
+-- ============================================================
+-- Allow anyone to read published posts
+-- ============================================================
+
+CREATE POLICY "Anyone can read published posts"
+  ON public.post_drafts FOR SELECT
+  USING (status = 'published');