fix: prevent email loop by detecting bounces and capping conversation length

- email_channel.py: add detect_automated_message() that inspects RFC 3834
  Auto-Submitted, mailer-daemon/postmaster sender patterns, X-Loop,
  multipart/report Content-Type, Precedence, and subject heuristics.
  fetch_unread_messages() now includes is_automated / automated_reason
  in every message dict.

- main.py: if is_automated is set, call agent.handle_automated_message()
  instead of process_message() — no reply is ever sent to a bounce source.

- agent/core.py: add MAX_USER_MESSAGES = 20 cap; process_message() returns
  "" without replying once a conversation exceeds the limit and calls
  notifier.notify_loop_escalation() on first breach.  New public method
  handle_automated_message() records the event and triggers the same
  one-shot admin alert.

- models/conversation.py: add loop_escalated: bool field (persisted) so
  the admin alert fires at most once per conversation.

- notifications/notifier.py: add notify_loop_escalation() which sends a
  plain-text warning to the admin CC list (Markus Graf / spielgruppen@).

https://claude.ai/code/session_01KwvR5hDPjSuJg4kvw5b5e5

src/models/conversation.py

@@ -35,6 +35,9 @@ class ConversationState:
     # Most recent inbound Message-ID — used for reply threading headers only,
     # NOT for conversation matching (which is always by email address).
     last_inbound_message_id: str = ""
+    # Loop / automated-sender prevention.
+    # Set to True once the admin has been notified; prevents repeated alerts.
+    loop_escalated: bool = False
 
     def to_dict(self) -> dict:
         return {
@@ -54,6 +57,7 @@ class ConversationState:
             "completed": self.completed,
             "reminder_count": self.reminder_count,
             "last_inbound_message_id": self.last_inbound_message_id,
+            "loop_escalated": self.loop_escalated,
         }
 
     @classmethod
@@ -78,4 +82,5 @@ class ConversationState:
         state.completed = data.get("completed", False)
         state.reminder_count = data.get("reminder_count", 0)
         state.last_inbound_message_id = data.get("last_inbound_message_id", "")
+        state.loop_escalated = data.get("loop_escalated", False)
         return state