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/database.py

@@ -150,6 +150,10 @@ def _migrate_upstream_generated_keys():
             conn.execute(text("UPDATE upstream_generated_keys SET updated_at = created_at WHERE updated_at IS NULL"))
         if "managed_prefix" not in columns:
             conn.execute(text("ALTER TABLE upstream_generated_keys ADD COLUMN managed_prefix VARCHAR(64)"))
+        if "imported_target_group_id" not in columns:
+            conn.execute(text("ALTER TABLE upstream_generated_keys ADD COLUMN imported_target_group_id VARCHAR(255)"))
+        if "imported_target_group_name" not in columns:
+            conn.execute(text("ALTER TABLE upstream_generated_keys ADD COLUMN imported_target_group_name VARCHAR(255)"))
 
     # ——— 历史数据迁移：回填 managed_prefix + 清理重复 ———
     with engine.begin() as conn: