Mail list: show who wrote, not everyone involved

The first column listed a thread's `participants`, which the threader builds from
from + to + cc — so every ordinary email read "Sender, You", and the +N badge
beside the avatar counted the user themselves.

`participants` cannot simply be narrowed: the subject-fallback join matches an
incoming message's recipients against it (threader._by_subject), so dropping
recipients would change how mail is threaded. Hence a separate `threads.senders`,
holding only from addresses, newest writer first — which is also the right thing
for a conversation where several people replied.

Migration 0019 backfills it, so existing threads do not wait to be touched again.
The list falls back to `participants` if a row somehow has no senders, rather
than rendering an empty column.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018SqJYt7jD7oUEkoY68sBZo

backend/app/api/schemas.py

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