feat(server): 归因v2——paid_social渠道与fb+付费medium优先规则

Co-Authored-By: Claude Code <noreply@anthropic.com>

server/tracker/tests/test_visitors.py

@@ -408,7 +408,7 @@ def test_detail_attribution_block(client, member_user):
     ft = attr["first_touch"]
     assert ft["channel"] == "natural_search" and ft["platform"] == "Bing"
     assert ft["search_term"] == "挖掘机" and ft["resolution"] == "resolved"
-    assert ft["evidence_kind"] == "observed_referrer" and ft["rule_version"] == "v1"
+    assert ft["evidence_kind"] == "observed_referrer" and ft["rule_version"] == "v2"
     assert ft["computed_at"] and ft["detail"] == {}
     assert [e["session_id"] for e in attr["session_entries"]] == ["s2", "s1"]  # 最近在前
     assert attr["session_entries"][0]["channel"] == "ai_referral"