fix(notify): switch Telegram parse_mode from Markdown to HTML

Underscores in alert text (dark_green, FIRE_BUY) broke Telegram's
legacy Markdown parser, causing ok:false → retries exhausted → failed.
HTML parse_mode is more robust and doesn't treat _ as italic.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/atm/notifier/telegram.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import html as _html
 from pathlib import Path
 from typing import Any
 
@@ -22,7 +23,7 @@ class TelegramNotifier:
         return _BASE.format(token=self._token, method=method)
 
     def send(self, alert: Alert) -> None:
-        text = f"*{alert.title}*\n{alert.body}"
+        text = f"<b>{_html.escape(alert.title)}</b>\n{_html.escape(alert.body)}"
 
         if alert.image_path and Path(alert.image_path).exists():
             with open(alert.image_path, "rb") as fh:
@@ -31,7 +32,7 @@ class TelegramNotifier:
                     data={
                         "chat_id": self._chat_id,
                         "caption": text,
-                        "parse_mode": "Markdown",
+                        "parse_mode": "HTML",
                     },
                     files={"photo": fh},
                     timeout=10,
@@ -42,7 +43,7 @@ class TelegramNotifier:
                 json={
                     "chat_id": self._chat_id,
                     "text": text,
-                    "parse_mode": "Markdown",
+                    "parse_mode": "HTML",
                 },
                 timeout=10,
             )

tests/test_notifier.py

@@ -228,7 +228,7 @@ def test_telegram_send_ok() -> None:
     n = TelegramNotifier("token", "chat123", session=session)
     n.send(_alert("Hi"))
     assert len(session.calls) == 1
-    assert "*Hi*" in session.calls[0]["json"]["text"]
+    assert "<b>Hi</b>" in session.calls[0]["json"]["text"]
 
 
 def test_telegram_429_raises() -> None: