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/threading_/threader.py

@@ -197,6 +197,16 @@ async def denormalize_thread(session: AsyncSession, thread_id: uuid.UUID) -> Non
                 seen_emails.add(email)
                 participants.append({"name": addr.get("name", ""), "email": addr.get("email", "")})
 
+    senders: list[dict[str, str]] = []
+    seen_senders: set[str] = set()
+    for msg in messages:  # newest first => most recent writer leads
+        for addr in msg.from_addr:
+            email = (addr.get("email") or "").lower()
+            if email and email not in seen_senders:
+                seen_senders.add(email)
+                senders.append({"name": addr.get("name", ""), "email": addr.get("email", "")})
+
+    thread.senders = senders
     thread.message_count = len(messages)
     thread.unread_count = sum(1 for m in messages if "\\Seen" not in m.flags)
     thread.has_attachments = any(m.has_attachments for m in messages)