fix(unsub): List-Unsubscribe 头兜底 + mailto 退订按钮放行

两封推广邮件无退订按钮，两个独立根因：
1. Cloudflare 邮件 (msg_id=1364)：程序只喂纯文本给 AI，纯文本里
   'Unsubscribe' 无 URL；邮件头 List-Unsubscribe 的权威退订链接被丢弃。
   -> 解析 List-Unsubscribe 头 (RFC 2369) 存入 Email.list_unsubscribe，
      注入 AI 提示词；显示层新增 _ensure_unsub_link 兜底：AI 漏提取时
      直接用头部 http(s)/mailto 链接补退订按钮（不依赖 AI）。
2. InferX 邮件 (msg_id=1363)：AI 提取到 mailto:team@inferx.net?subject=Unsubscribe，
   但 _link_buttons 只放行 http 协议把它过滤掉。
   -> URL 校验放行 mailto:（退订场景常见）。

send_summary / tg_send_and_mark 均应用兜底并写回 summary_data.links，
回调恢复视图也能显示退订按钮。新增 6 个测试，共 14 个全部通过。

tests/test_unsub_keyboard.py

@@ -1,6 +1,11 @@
 """测试摘要键盘渲染：推广邮件的退订链接按钮必须显示且排最前。"""
 
-from src.tg_bot import _is_unsub_link, _link_buttons, _summary_keyboard
+from src.tg_bot import (
+    _ensure_unsub_link,
+    _is_unsub_link,
+    _link_buttons,
+    _summary_keyboard,
+)
 
 
 def _rows_of(kb: dict) -> list[list[dict]]:
@@ -102,4 +107,43 @@ def test_oversized_url_filtered():
     url = "https://example.com/" + "x" * 2100
     links = [{"text": "退订", "url": url}]
     rows = _link_buttons(links)
-    assert rows == []
+    assert rows == []
+
+
+def test_mailto_unsub_link_rendered():
+    """mailto: 退订链接（如 InferX 的 mailto:team@inferx.net?subject=Unsubscribe）必须渲染"""
+    links = [{"text": "退订", "url": "mailto:team@inferx.net?subject=Unsubscribe"}]
+    kb = _summary_keyboard(can_reply=False, is_promotion=True, links=links)
+    rows = kb["inline_keyboard"]
+    assert rows[0][0]["url"] == "mailto:team@inferx.net?subject=Unsubscribe"
+    assert "🔕" in rows[0][0]["text"]
+    assert _is_unsub_link(links[0])
+
+
+def test_ensure_unsub_link_http_header_fallback():
+    """AI 没提取到退订链接时，用 List-Unsubscribe 头的 http 链接兜底"""
+    header = "https://links.iterable.com/s/uh/tKjpy4SHCKvbzE2votNyZEJDvlMdeodVyWFExNs310ugJ_RvZtoZq4gGLQ0BXMSL2lf3kEZsmr1XJmrODIFHpEwcypo3LR096pZ7jsfBynZBgGWz171Wc1E6ZIIC5Lu0e-Stq6p5-_Ad3CnNlUWrW1tnrLSeiGsOiQG_zMYKVbtfQZgvfm3QYfD9lQ/Cey29KU8yOzIUltaEJA8EMRFf-FKMZQy/24"
+    links = _ensure_unsub_link([], header)
+    assert links == [{"text": "退订", "url": header}]
+    kb = _summary_keyboard(can_reply=False, is_promotion=True, links=links)
+    assert kb["inline_keyboard"][0][0]["url"] == header
+
+
+def test_ensure_unsub_link_mailto_header_fallback():
+    """List-Unsubscribe 头只有 mailto 时也兜底"""
+    header = "mailto:unsubscribe+19693436+25822730@unsubscribe.iterable.com"
+    links = _ensure_unsub_link([], header)
+    assert links == [{"text": "退订", "url": header}]
+
+
+def test_ensure_unsub_link_no_duplicate_when_ai_extracted():
+    """AI 已提取退订链接时，不重复添加头部链接"""
+    ai_links = [{"text": "取消订阅", "url": "https://r.openai.com/asm/unsubscribe/?x=1"}]
+    links = _ensure_unsub_link(ai_links, "https://links.example.com/unsub")
+    assert links == ai_links
+
+
+def test_ensure_unsub_link_ignores_invalid_header():
+    """头部无有效链接时不兜底"""
+    assert _ensure_unsub_link([], "") == []
+    assert _ensure_unsub_link([], "javascript:alert(1)") == []