feat(backend): 无界列表端点加分页——projects/templates 支持 limit/offset

GET /projects 与 GET /templates 原一次返回全部 owner 行，行数随数据线性增长。
新增可复用分页依赖 ww_api.pagination.get_page（limit∈[1,200] 默认 50、offset≥0，
越界 → 422），两端点接入，Repository.list_for_owner 加 keyword-only limit/offset
把 LIMIT/OFFSET 下推 DB。向后兼容：不传参返回第一页。补 projects 分页 + 边界 422 测试。

packages/core/tests/test_template_repo.py

@@ -38,8 +38,13 @@ class _FakeTemplateRepo:
         self.flushed += 1
         return view
 
-    async def list_for_owner(self, owner_id: uuid.UUID) -> list[TemplateView]:
-        return [v for tid, v in self.rows.items() if self.owners[tid] == owner_id]
+    async def list_for_owner(
+        self, owner_id: uuid.UUID, *, limit: int | None = None, offset: int = 0
+    ) -> list[TemplateView]:
+        views = [v for tid, v in self.rows.items() if self.owners[tid] == owner_id]
+        if limit is None:
+            return views
+        return views[offset : offset + limit]
 
     async def delete(self, owner_id: uuid.UUID, template_id: uuid.UUID) -> bool:
         if self.rows.get(template_id) is None or self.owners.get(template_id) != owner_id: