feat(assets): wire cursor pagination through GET /api/assets handler

Adds integration tests for: full cursor walk, invalid-cursor 400,
sort/cursor mismatch 400, cursor-wins-over-offset, absent next_cursor
when no more results, and pagination stability across deletes.

app/assets/api/routes.py

@@ -39,6 +39,7 @@ from app.assets.services import (
     update_asset_metadata,
     upload_from_temp_path,
 )
+from app.assets.services.cursor import InvalidCursorError
 from app.assets.services.tagging import list_tag_histogram
 
 ROUTES = web.RouteTableDef()
@@ -209,24 +210,40 @@ async def list_assets_route(request: web.Request) -> web.Response:
     order_candidate = (q.order or "desc").lower()
     order = order_candidate if order_candidate in {"asc", "desc"} else "desc"
 
-    result = list_assets_page(
-        owner_id=USER_MANAGER.get_request_user_id(request),
-        include_tags=q.include_tags,
-        exclude_tags=q.exclude_tags,
-        name_contains=q.name_contains,
-        metadata_filter=q.metadata_filter,
-        limit=q.limit,
-        offset=q.offset,
-        sort=sort,
-        order=order,
-    )
+    try:
+        result = list_assets_page(
+            owner_id=USER_MANAGER.get_request_user_id(request),
+            include_tags=q.include_tags,
+            exclude_tags=q.exclude_tags,
+            name_contains=q.name_contains,
+            metadata_filter=q.metadata_filter,
+            limit=q.limit,
+            offset=q.offset,
+            sort=sort,
+            order=order,
+            after=q.after,
+        )
+    except InvalidCursorError as e:
+        return web.json_response(
+            {"error": {"code": "INVALID_CURSOR", "message": str(e)}},
+            status=400,
+        )
 
     summaries = [_build_asset_response(item) for item in result.items]
 
+    # has_more semantics differ by mode:
+    #   - cursor mode: a non-empty next_cursor means there are more results.
+    #   - offset mode: derived from total - (offset + page size).
+    if q.after is not None:
+        has_more = result.next_cursor is not None
+    else:
+        has_more = (q.offset + len(summaries)) < result.total
+
     payload = schemas_out.AssetsList(
         assets=summaries,
         total=result.total,
-        has_more=(q.offset + len(summaries)) < result.total,
+        has_more=has_more,
+        next_cursor=result.next_cursor,
     )
     return web.json_response(payload.model_dump(mode="json", exclude_none=True))
 

tests-unit/assets_test/test_list_cursor.py

@@ -0,0 +1,189 @@
+"""Integration tests for cursor-based pagination on GET /api/assets.
+
+Wire contract is shared with cloud's Go implementation (BE-893). These tests
+exercise the handler/service/query path end-to-end; cursor-encoding-level
+tests live in tests-unit/assets_test/services/test_cursor.py.
+"""
+import requests
+
+
+def _seed(asset_factory, make_asset_bytes, count: int, tag: str) -> list[str]:
+    names = [f"cursor_{i:02d}.safetensors" for i in range(count)]
+    for n in names:
+        asset_factory(
+            n,
+            ["models", "checkpoints", "unit-tests", tag],
+            {},
+            make_asset_bytes(n, size=2048),
+        )
+    return sorted(names)
+
+
+def test_cursor_pages_all_items_in_order(http: requests.Session, api_base: str, asset_factory, make_asset_bytes):
+    names = _seed(asset_factory, make_asset_bytes, count=5, tag="cursor-walk")
+
+    params = {
+        "include_tags": "unit-tests,cursor-walk",
+        "sort": "name",
+        "order": "asc",
+        "limit": "2",
+    }
+
+    seen: list[str] = []
+    after: str | None = None
+    pages = 0
+    while True:
+        page_params = dict(params)
+        if after is not None:
+            page_params["after"] = after
+        r = http.get(api_base + "/api/assets", params=page_params, timeout=120)
+        assert r.status_code == 200, r.text
+        body = r.json()
+        seen.extend(a["name"] for a in body["assets"])
+        pages += 1
+        after = body.get("next_cursor")
+        if after is None:
+            break
+        assert body["has_more"] is True
+        assert pages < 10, "guard against runaway cursor loop"
+
+    assert seen == names, f"expected {names}, got {seen}"
+    # Last page should have has_more False
+    assert body["has_more"] is False
+    assert "next_cursor" not in body
+
+
+def test_cursor_invalid_returns_400(http: requests.Session, api_base: str):
+    r = http.get(
+        api_base + "/api/assets",
+        params={"after": "not-a-real-cursor", "sort": "created_at"},
+        timeout=120,
+    )
+    assert r.status_code == 400, r.text
+    body = r.json()
+    assert body["error"]["code"] == "INVALID_CURSOR"
+
+
+def test_cursor_sort_mismatch_returns_400(http: requests.Session, api_base: str, asset_factory, make_asset_bytes):
+    _seed(asset_factory, make_asset_bytes, count=2, tag="cursor-mismatch")
+
+    # Take a real cursor minted for sort=name.
+    r = http.get(
+        api_base + "/api/assets",
+        params={
+            "include_tags": "unit-tests,cursor-mismatch",
+            "sort": "name",
+            "order": "asc",
+            "limit": "1",
+        },
+        timeout=120,
+    )
+    assert r.status_code == 200
+    cursor = r.json()["next_cursor"]
+    assert cursor is not None
+
+    # Replay against sort=created_at — should fail with INVALID_CURSOR.
+    r2 = http.get(
+        api_base + "/api/assets",
+        params={"after": cursor, "sort": "created_at"},
+        timeout=120,
+    )
+    assert r2.status_code == 400, r2.text
+    assert r2.json()["error"]["code"] == "INVALID_CURSOR"
+
+
+def test_cursor_wins_over_offset(http: requests.Session, api_base: str, asset_factory, make_asset_bytes):
+    names = _seed(asset_factory, make_asset_bytes, count=4, tag="cursor-vs-offset")
+
+    # Take a cursor that points past the first item.
+    r = http.get(
+        api_base + "/api/assets",
+        params={
+            "include_tags": "unit-tests,cursor-vs-offset",
+            "sort": "name",
+            "order": "asc",
+            "limit": "1",
+        },
+        timeout=120,
+    )
+    cursor = r.json()["next_cursor"]
+    assert cursor is not None
+
+    # Pass both 'after' and a large offset. Cursor must win; offset is ignored.
+    r2 = http.get(
+        api_base + "/api/assets",
+        params={
+            "include_tags": "unit-tests,cursor-vs-offset",
+            "sort": "name",
+            "order": "asc",
+            "limit": "1",
+            "after": cursor,
+            "offset": "999",
+        },
+        timeout=120,
+    )
+    assert r2.status_code == 200
+    body = r2.json()
+    # Should land on the second name in sorted order — not skip ahead by 999.
+    assert [a["name"] for a in body["assets"]] == [names[1]]
+
+
+def test_next_cursor_absent_when_no_more_results(http: requests.Session, api_base: str, asset_factory, make_asset_bytes):
+    _seed(asset_factory, make_asset_bytes, count=2, tag="cursor-exhaust")
+
+    r = http.get(
+        api_base + "/api/assets",
+        params={
+            "include_tags": "unit-tests,cursor-exhaust",
+            "sort": "name",
+            "order": "asc",
+            "limit": "50",
+        },
+        timeout=120,
+    )
+    body = r.json()
+    assert body["has_more"] is False
+    assert "next_cursor" not in body
+
+
+def test_cursor_pagination_stable_after_delete(http: requests.Session, api_base: str, asset_factory, make_asset_bytes):
+    names = _seed(asset_factory, make_asset_bytes, count=4, tag="cursor-delete")
+
+    # Page 1.
+    r = http.get(
+        api_base + "/api/assets",
+        params={
+            "include_tags": "unit-tests,cursor-delete",
+            "sort": "name",
+            "order": "asc",
+            "limit": "2",
+        },
+        timeout=120,
+    )
+    assert r.status_code == 200
+    body = r.json()
+    page1_names = [a["name"] for a in body["assets"]]
+    cursor = body["next_cursor"]
+    assert cursor is not None
+    assert page1_names == names[:2]
+
+    # Delete an item from page 1 (already returned) — cursor should still
+    # locate the next page from where it was minted, not re-index.
+    target_id = body["assets"][0]["id"]
+    d = http.delete(api_base + f"/api/assets/{target_id}", timeout=120)
+    assert d.status_code in (200, 204), d.text
+
+    # Page 2 via cursor.
+    r2 = http.get(
+        api_base + "/api/assets",
+        params={
+            "include_tags": "unit-tests,cursor-delete",
+            "sort": "name",
+            "order": "asc",
+            "limit": "2",
+            "after": cursor,
+        },
+        timeout=120,
+    )
+    body2 = r2.json()
+    assert [a["name"] for a in body2["assets"]] == names[2:]