feat: rewrite cover generation — 480×800 portrait, themed AI backgrounds, two-layer pipeline

Made-with: Cursor

src/routes/publish.py

@@ -74,9 +74,14 @@ def create_issue():
         .order_by(Article.pub_date.asc()).all()
     ]
 
+    categories_list = []
+    for a in Article.query.filter(Article.id.in_(included_ids)).all():
+        categories_list.extend(json.loads(a.categories))
+
     try:
         cover_path = generate_cover(
-            cover_method, config.ISSUES_DIR, week_start, week_end, headlines
+            cover_method, config.ISSUES_DIR, week_start, week_end,
+            headlines, categories_list
         )
         epub_path = build_epub(
             week_start, week_end, included_ids, cover_path, config.ISSUES_DIR