fix(ad): 服务端强制不足一秒曝光收益为零 (#150)

## 改动
- eCPM 上报新增可选 exposure_ms，兼容旧客户端
- exposure_ms < 1000 时保留展示记录并强制有效 eCPM 为 0
- 失败领券任务允许保留短曝光零收益 trace，后台显示 0 而不是未填充
- 其他失败后的迟到曝光仍按原规则解绑 trace

## 验证
- 相关 pytest：10 passed
- Ruff：通过
- compileall：通过

依赖：先合并 Server #149。

---------

Co-authored-by: unknown <798648091@qq.com>
Reviewed-on: #150
Co-authored-by: linkeyu <linkeyu@wonderable.ai>
Co-committed-by: linkeyu <linkeyu@wonderable.ai>

tests/test_ad_ecpm_trace_revenue.py

@@ -58,6 +58,32 @@ def test_revenue_yuan_by_trace_empty() -> None:
         db.close()
 
 
+def test_short_exposure_keeps_record_with_zero_revenue() -> None:
+    """不足一秒仍落展示记录，以便后台显示 0 而不是未填充。"""
+    db = SessionLocal()
+    try:
+        rec = crud_ecpm.create_ecpm_record(
+            db, 1, ad_type="draw", ecpm_raw="350",
+            ad_session_id="sess-short-exposure", feed_scene="coupon",
+            trace_id="trace-short-exposure", exposure_ms=999,
+        )
+        assert rec.ecpm_raw == "0"
+        assert crud_ecpm.revenue_yuan_by_trace(db, ["trace-short-exposure"]) == {
+            "trace-short-exposure": 0.0
+        }
+    finally:
+        db.execute(delete(AdEcpmRecord).where(
+            AdEcpmRecord.ad_session_id == "sess-short-exposure"
+        ))
+        db.commit()
+        db.close()
+
+
+def test_one_second_exposure_keeps_original_ecpm() -> None:
+    assert crud_ecpm.effective_ecpm_raw("350", 1000) == "350"
+    assert crud_ecpm.effective_ecpm_raw("350", None) == "350"
+
+
 def test_terminal_coupon_trace_is_not_attributable_to_late_impression() -> None:
     """领券失败或被放弃后才到达的广告展示保留收益记录，但不再关联死亡 trace。"""
     db = SessionLocal()
@@ -80,6 +106,12 @@ def test_terminal_coupon_trace_is_not_attributable_to_late_impression() -> None:
         assert crud_ecpm.attributable_trace_id(
             db, feed_scene="coupon", trace_id="abandoned-before-ad"
         ) is None
+        assert crud_ecpm.attributable_trace_id(
+            db, feed_scene="coupon", trace_id="failed-before-ad", exposure_ms=999
+        ) == "failed-before-ad"
+        assert crud_ecpm.attributable_trace_id(
+            db, feed_scene="coupon", trace_id="abandoned-before-ad", exposure_ms=999
+        ) == "abandoned-before-ad"
         assert crud_ecpm.attributable_trace_id(
             db, feed_scene="comparison", trace_id="failed-before-ad"
         ) == "failed-before-ad"