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/test_projects.py

@@ -222,6 +222,33 @@ async def test_list_projects() -> None:
     assert all(p["pending_review_count"] == 0 for p in projects)
 
 
+@pytest.mark.asyncio
+async def test_list_projects_paginates_with_limit_and_offset() -> None:
+    client, _, _, _ = _make_client()
+    async with client:
+        for t in ("甲", "乙", "丙"):
+            await client.post("/projects", json={"title": t})
+        page1 = await client.get("/projects?limit=2")
+        page2 = await client.get("/projects?limit=2&offset=2")
+    assert page1.status_code == 200
+    assert [p["title"] for p in page1.json()["projects"]] == ["甲", "乙"]
+    assert page2.status_code == 200
+    assert [p["title"] for p in page2.json()["projects"]] == ["丙"]
+
+
+@pytest.mark.asyncio
+async def test_list_projects_rejects_out_of_range_pagination() -> None:
+    client, _, _, _ = _make_client()
+    async with client:
+        too_small = await client.get("/projects?limit=0")
+        too_large = await client.get("/projects?limit=201")
+        neg_offset = await client.get("/projects?offset=-1")
+    # 边界越界 → FastAPI 422（limit∈[1,200]、offset≥0）。
+    assert too_small.status_code == 422
+    assert too_large.status_code == 422
+    assert neg_offset.status_code == 422
+
+
 @pytest.mark.asyncio
 async def test_get_project_detail() -> None:
     client, _, _, _ = _make_client()