fix(compare): coupon_name 端到端透传 + 兑换下限正向测试

代码评审(server#12 / pricebot#38)修复:
- ComparisonResultIn / comparison 模型 / DB·API 文档补 coupon_name(优惠来源名),
  否则 pydantic 在上报边界静默丢弃,记录页来源 chip 落不了地;扩展 passthrough 测试覆盖
- 新增最小额兑换正向用例,钉死 MIN_EXCHANGE_COIN=100(可兑 1 分)新下限

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_compare_record.py

@@ -33,7 +33,8 @@ def _food_payload(trace_id: str) -> dict:
         ],
         "comparison_results": [
             {"platform_id": "meituan", "platform_name": "美团", "package": "com.sankuai.meituan",
-             "price": 123.50, "is_source": False, "rank": 1, "coupon_saved": 7.0},
+             "price": 123.50, "is_source": False, "rank": 1,
+             "coupon_saved": 7.0, "coupon_name": "外卖大额神券"},
             {"platform_id": "taobao_flash", "platform_name": "淘宝闪购", "package": "com.taobao.taobao",
              "price": 128.50, "is_source": True, "rank": 2},
             {"platform_id": "jd_waimai", "platform_name": "京东外卖", "package": "com.jingdong.app.mall",
@@ -79,7 +80,7 @@ def test_report_and_derive(client) -> None:
 
 
 def test_coupon_saved_passthrough(client) -> None:
-    """红包优惠额 coupon_saved 上报→落库→读出;仅目标平台带值,源平台不带(null)。"""
+    """红包优惠额 coupon_saved + 来源名 coupon_name 上报→落库→读出;仅目标平台带值,源平台不带(null)。"""
     token = _login(client, "13800002010")
     rid = client.post(
         "/api/v1/compare/record", json=_food_payload("trace-coupon"), headers=_auth(token)
@@ -87,7 +88,9 @@ def test_coupon_saved_passthrough(client) -> None:
     d = client.get(f"/api/v1/compare/records/{rid}", headers=_auth(token)).json()
     by_pid = {x["platform_id"]: x for x in d["comparison_results"]}
     assert by_pid["meituan"]["coupon_saved"] == 7.0            # 目标平台带值
+    assert by_pid["meituan"]["coupon_name"] == "外卖大额神券"   # 来源名透传(pricebot#38)
     assert by_pid["taobao_flash"].get("coupon_saved") is None  # 源平台不带/为 null
+    assert by_pid["taobao_flash"].get("coupon_name") is None   # 源平台无来源名
 
 
 def test_source_is_cheapest_no_saving(client) -> None:

tests/test_welfare.py

@@ -180,6 +180,51 @@ def test_exchange_flow(client) -> None:
     assert page["items"][0]["biz_type"] == "exchange_in"
 
 
+def test_exchange_min_floor_one_cent(client) -> None:
+    """锁定本次下调的兑换下限:正好兑下限额(MIN_EXCHANGE_COIN=100 金币=1 分)应成功。
+
+    这是本 PR 的核心新能力(下限 10000→100,可兑 1 分起)。test_exchange_flow 兑的是
+    1 元(10000),在旧下限下也通过,证明不了新下限;本用例供款并兑换正好等于下限的
+    100 金币,断言到账 1 分。若有人把 MIN_EXCHANGE_COIN 改回 10000,100 会因低于下限被
+    判 400,本用例随之失败,从而把新下限值钉死。
+    """
+    phone = "13800001010"
+    token = _login(client, phone)
+    # 供款: 正好注入一个下限额度的金币(=COIN_PER_CENT=100)
+    with SessionLocal() as db:
+        user = get_user_by_phone(db, phone)
+        assert user is not None
+        crud_wallet.grant_coins(
+            db, user.id, MIN_EXCHANGE_COIN, biz_type="test_seed", remark="测试供款"
+        )
+        db.commit()
+
+    # 兑换下限额 100 金币 → 1 分
+    r = client.post(
+        "/api/v1/wallet/exchange",
+        json={"coin_amount": MIN_EXCHANGE_COIN},
+        headers=_auth(token),
+    )
+    assert r.status_code == 200, r.text
+    res = r.json()
+    assert res["coin_amount"] == MIN_EXCHANGE_COIN
+    assert res["cash_added_cents"] == 1  # coins_to_cents(100) == 1
+    assert res["coin_balance"] == 0
+    assert res["cash_balance_cents"] == 1
+
+    # 金币流水有一笔 exchange_out(=-100)
+    r = client.get("/api/v1/wallet/coin-transactions", headers=_auth(token))
+    out_txn = next(t for t in r.json()["items"] if t["biz_type"] == "exchange_out")
+    assert out_txn["amount"] == -MIN_EXCHANGE_COIN
+
+    # 现金流水有且仅有一笔 exchange_in(=+1 分)
+    r = client.get("/api/v1/wallet/cash-transactions", headers=_auth(token))
+    cash = r.json()["items"]
+    assert len(cash) == 1
+    assert cash[0]["amount_cents"] == 1
+    assert cash[0]["biz_type"] == "exchange_in"
+
+
 def test_exchange_insufficient_and_invalid(client) -> None:
     token = _login(client, "13800001006")