Revert "Mail list: show who wrote, not everyone involved"

This reverts commit 550b8cf503.

backend/app/api/schemas.py

@@ -17,7 +17,6 @@ class ThreadSummary(BaseModel):
     id: uuid.UUID
     subject: str
     snippet: str
-    senders: list[Address]
     participants: list[Address]
     account_ids: list[uuid.UUID]
     message_count: int

backend/app/api/threads.py

@@ -38,7 +38,6 @@ def _summary(thread: Thread, message_count: int | None = None) -> ThreadSummary:
         id=thread.id,
         subject=thread.subject_norm or "(no subject)",
         snippet=thread.snippet,
-        senders=_addresses(thread.senders),
         participants=_addresses(thread.participants),
         account_ids=list(thread.account_ids),
         # Count only the messages this view actually shows (deleted/spam excluded