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。

apps/api/tests/fakes_projects.py

@@ -63,8 +63,13 @@ class FakeProjectRepo:
         self.rows[pid] = (owner_id, view)
         return view
 
-    async def list_for_owner(self, owner_id: uuid.UUID) -> list[ProjectView]:
-        return [v for (o, v) in self.rows.values() if o == owner_id]
+    async def list_for_owner(
+        self, owner_id: uuid.UUID, *, limit: int | None = None, offset: int = 0
+    ) -> list[ProjectView]:
+        views = [v for (o, v) in self.rows.values() if o == owner_id]
+        if limit is None:
+            return views
+        return views[offset : offset + limit]
 
     async def get(self, owner_id: uuid.UUID, project_id: uuid.UUID) -> ProjectView | None:
         entry = self.rows.get(project_id)