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/ww_api/routers/projects.py

@@ -46,6 +46,7 @@ from ww_llm_gateway import Gateway
 from ww_shared import AppError, ErrorCode, ErrorEnvelope
 
 from ww_api.logging_config import get_logger
+from ww_api.pagination import PageDep
 from ww_api.schemas.injection import (
     InjectionEntity,
     InjectionEntityRef,
@@ -144,8 +145,9 @@ async def create_project(body: ProjectCreateRequest, repo: ProjectRepoDep) -> Pr
 
 
 @router.get("")
-async def list_projects(repo: ProjectRepoDep) -> ProjectListResponse:
-    views = await repo.list_for_owner(STUB_OWNER_ID)
+async def list_projects(repo: ProjectRepoDep, page: PageDep) -> ProjectListResponse:
+    """作品列表（按 created_at 升序，分页）。`?limit=&offset=`，默认第一页。"""
+    views = await repo.list_for_owner(STUB_OWNER_ID, limit=page.limit, offset=page.offset)
     return ProjectListResponse(projects=[_to_response(v) for v in views])