fix(priority-sync): handle missing rate data and backfill target group on re-import

P1 - Missing rate data now skips account instead of falling back to 1.0:
  In sync_account_priorities_for_upstream(), the rated list now filters
  out accounts whose upstream snapshot has no rate entry for their group_id.
  If after filtering a competitive bucket has fewer than 2 accounts with
  valid rate data, the entire bucket is silently skipped (no update_account
  call, no webhook) rather than treating missing rates as 1.0 and
  potentially triggering spurious notifications.

P2 - Re-importing an existing account now backfills imported_target_group_id:
  In the exists-is-True idempotency branch of import_upstream_keys_as_accounts(),
  if the current request supplies a target_group_id for the account's source group
  and it differs from what is stored, the field is written back and committed.
  This lets operators fix old data by simply re-running the import dialog.

Tests added:
  - test_missing_rate_skips_entire_competitive_group: all accounts in
    competitive group lack snapshot → bucket skipped, no update called
  - test_partial_missing_rate_sufficient_accounts_still_updates: 3 accounts
    in same bucket, 1 missing rate → the 2 with rates still compete normally

All 27 tests pass.

backend/app/routers/websites.py

@@ -558,6 +558,11 @@ 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:
+                    # 顺手回填 imported_target_group_id（老数据升级后可通过重导自动补齐）
+                    new_tgid = body.target_group_map.get(row.group_id) or None
+                    if new_tgid and row.imported_target_group_id != new_tgid:
+                        row.imported_target_group_id = new_tgid
+                        db.commit()
                     items.append(ImportAccountItem(
                         upstream_key_id=row.id,
                         source_group_id=row.group_id,