feat(notifier): Alert.silent + TelegramNotifier disable_notification

Silent screenshots for periodic auto-poll — Telegram param
disable_notification=True suppresses phone notification sound.
Discord ignores the field (no equivalent).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/atm/notifier/__init__.py

@@ -5,11 +5,13 @@ from typing import Protocol
 
 @dataclass
 class Alert:
-    kind: str          # "trigger" | "heartbeat" | "levels" | "warn" | "arm" | "prime" | "late_start"
+    # flat union: "trigger"|"heartbeat"|"levels"|"warn"|"arm"|"prime"|"late_start"|"screenshot"|"status"
+    kind: str
     title: str
     body: str
     image_path: Path | None = None  # annotated screenshot
     direction: str | None = None    # "BUY"/"SELL" when kind=trigger
+    silent: bool = False            # disable_notification for Telegram; ignored by Discord
 
 
 class Notifier(Protocol):

src/atm/notifier/telegram.py

@@ -33,6 +33,7 @@ class TelegramNotifier:
                         "chat_id": self._chat_id,
                         "caption": text,
                         "parse_mode": "HTML",
+                        "disable_notification": str(alert.silent).lower(),
                     },
                     files={"photo": fh},
                     timeout=10,
@@ -44,6 +45,7 @@ class TelegramNotifier:
                     "chat_id": self._chat_id,
                     "text": text,
                     "parse_mode": "HTML",
+                    "disable_notification": alert.silent,
                 },
                 timeout=10,
             )