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

app/api/v1/invite.py

@@ -38,7 +38,7 @@ logger = logging.getLogger("shagua.invite")
 router = APIRouter(prefix="/api/v1/invite", tags=["invite"])
 
 _BIND_MESSAGES = {
-    "success": "邀请绑定成功,金币已到账",
+    "success": "邀请绑定成功",
     "already_bound": "你已绑定过邀请人",
     "invalid_code": "邀请码无效",
     "self_invite": "不能填写自己的邀请码",
@@ -161,7 +161,7 @@ def landing_track(
     return LandingTrackOut(status="ok")
 
 
-@router.post("/bind", response_model=BindInviteOut, summary="绑定邀请人(注册即生效,双方发金币)")
+@router.post("/bind", response_model=BindInviteOut, summary="绑定邀请人(注册即生效,绑定不发奖)")
 def bind_invite(
     req: BindInviteIn, user: CurrentUser, db: DbSession, request: Request
 ) -> BindInviteOut: