feat(invite): 发奖规则 v2 改 v3 — 绑定双方都不发钱

- 绑定不再给被邀请人发新人金币(去掉拉新即时激励); 双方绑定时都不发钱、只建归因关系
- 邀请人收益仍走"好友成功比价后发 2 元邀请奖励金"(不变, 见 try_reward_on_compare)
- 删 INVITE_INVITEE_COINS / INVITE_INVITER_COINS 常量; 绑定成功文案去掉"金币已到账"
- 跟进测试: test_invite 断言绑定双方金币=0; 修 test_welfare 账户接口补 invite_cash_balance_cents 断言

tests/test_welfare.py

@@ -46,7 +46,12 @@ def test_account_auto_created_empty(client) -> None:
     r = client.get("/api/v1/wallet/account", headers=_auth(token))
     assert r.status_code == 200, r.text
     body = r.json()
-    assert body == {"coin_balance": 0, "cash_balance_cents": 0, "total_coin_earned": 0}
+    assert body == {
+        "coin_balance": 0,
+        "cash_balance_cents": 0,
+        "invite_cash_balance_cents": 0,    # v2 账户隔离新增(邀请奖励金,与现金隔离)
+        "total_coin_earned": 0,
+    }
 
 
 def test_signin_flow(client) -> None: