fix(priority-sync): narrow account priority update to competitive groups only

Root cause: sync_account_priorities_for_upstream() was doing a global
priority re-rank across ALL imported accounts on a website whenever any
upstream rate changed, triggering spurious account_priority_changed
notifications for accounts in different target groups with no competition.

Fix:
- Add imported_target_group_id / imported_target_group_name to
  UpstreamGeneratedKey (nullable; old data falls back to group_id)
- Writ imported_target_group_id on account import in websites.py
- Rewrite sync_account_priorities_for_upstream():
  * bucket accounts by competition_group = imported_target_group_id or group_id
  * only process buckets with count > 1 (genuine competition)
  * each competitive bucket independently sorted by rate; priority starts at 1
  * single-account groups: completely skipped (no update_account, no notification)
  * no competitive groups at all: early return, no log, no notification
- Remove auto priority update in re-import idempotency path (was also
  incorrect; now fully delegated to sync_account_priorities_for_upstream)
- Fix Sub2ApiWebsiteClient local import in sync fn → use module-level name
  so monkeypatch works correctly in tests

Tests: rewrite test_priority_sync.py
- REMOVED: test_priority_sync_full_website_update (was asserting the buggy behavior)
- NEW: test_no_update_when_different_groups_single_account_each
- NEW: test_same_target_group_two_accounts_updated
- NEW: test_two_target_groups_independent_priority
- NEW: test_old_data_null_target_group_fallback
- NEW: test_single_account_in_mixed_website
- UPDATED: test_priority_sync_log_structure (now requires competitive group)
- KEPT: test_priority_sync_cross_upstream_group, test_import_auto_priority_by_rate

All 25 tests pass (8 priority_sync + 17 existing upstream tests).

backend/app/routers/websites.py

@@ -558,16 +558,6 @@ def import_upstream_keys_as_accounts(
                 old_account_id = row.imported_account_id
                 exists = c.account_exists(row.imported_account_id)
                 if exists is True:
-                    # 自动更新已有账号的 priority（分步导入时全局倍率排序可能已变）
-                    new_priority = rate_priority_map.get(f"{row.upstream_id}:{row.group_id}") if body.auto_priority_by_rate else None
-                    priority_msg = "已导入过，已跳过"
-                    if new_priority is not None:
-                        try:
-                            c.update_account(old_account_id, {"priority": new_priority})
-                            priority_msg = f"已导入过，优先级已更新为 {new_priority}"
-                        except Exception as exc:
-                            logger.warning("update priority failed account=%s: %s", old_account_id, exc)
-                            priority_msg = f"已导入过，优先级更新失败: {exc}"
                     items.append(ImportAccountItem(
                         upstream_key_id=row.id,
                         source_group_id=row.group_id,
@@ -578,7 +568,7 @@ def import_upstream_keys_as_accounts(
                         platform=platform,
                         upstream_base_url=upstream_base_url,
                         status="exists",
-                        message=priority_msg,
+                        message="已导入过，已跳过",
                     ))
                     continue
                 elif exists is False:
@@ -639,6 +629,8 @@ def import_upstream_keys_as_accounts(
                 row.imported_website_id = wid
                 row.imported_account_id = account_id or None
                 row.imported_at = datetime.now(timezone.utc)
+                row.imported_target_group_id = target_group_id or None
+                row.imported_target_group_name = None  # target_group_map 只存 ID，name 展示用可留 NULL
                 row.status = "imported"
                 row.error = None
                 db.commit()