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/models.py

@@ -152,6 +152,13 @@ class Thread(Base):
     )
     subject_norm: Mapped[str] = mapped_column(Text, nullable=False)
     snippet: Mapped[str] = mapped_column(Text, nullable=False, server_default=text("''"))
+    # Who WROTE in this thread, newest first. Distinct from `participants`, which
+    # also holds recipients because the subject-fallback threading matches on them
+    # (threader._by_subject). The message list shows senders; conflating the two
+    # put "you" next to every sender in the list.
+    senders: Mapped[list[dict[str, str]]] = mapped_column(
+        JSONB, nullable=False, server_default=text("'[]'")
+    )
     participants: Mapped[list[dict[str, str]]] = mapped_column(
         JSONB, nullable=False, server_default=text("'[]'")
     )