Mail list: name everyone who wrote in the thread, and call the user "me"

The sender column showed participants[0], and participants are rebuilt
newest message first — so replying to a thread put the user in that slot
and the list named them instead of the person who wrote in.

Thread summaries (list and search) now carry `senders`: the distinct From
addresses of the thread's visible messages, oldest first, with the same
Trash/Spam visibility as the message count. The list joins them and shows
the user's own account addresses as "me" ("me, Alice" for a thread the
user started), and the avatar prefers the other side of the conversation.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/tests/api/test_api.py

@@ -37,11 +37,17 @@ class ApiCtx:
 
 
 def _raw(
-    subject: str, body: str, *, mid: str, refs: str | None = None, html: str | None = None
+    subject: str,
+    body: str,
+    *,
+    mid: str,
+    refs: str | None = None,
+    html: str | None = None,
+    sender: str = "Alice <alice@example.com>",
 ) -> bytes:
     m = EmailMessage()
     m["Subject"] = subject
-    m["From"] = "Alice <alice@example.com>"
+    m["From"] = sender
     m["To"] = "me@example.com"
     m["Message-ID"] = mid
     m["Date"] = "Mon, 09 Mar 2026 08:36:37 +0100"
@@ -159,6 +165,40 @@ async def test_list_threads(api: ApiCtx) -> None:
     assert kickoff["message_count"] == 2
 
 
+async def test_list_senders_are_authors_oldest_first(api: ApiCtx) -> None:
+    # Our reply joins the kickoff thread: the sender column lists both authors in
+    # the order they wrote — Alice twice collapses to one entry, and the recipients
+    # (me, on Alice's messages) are not senders.
+    assert api.session is not None and api.account_id is not None
+    inbox = await api.session.scalar(
+        select(Mailbox).where(Mailbox.account_id == api.account_id, Mailbox.path == "INBOX")
+    )
+    assert inbox is not None
+    raw = _raw(
+        "Re: Project kickoff",
+        "thanks",
+        mid="<d@x.com>",
+        refs="<b@x.com>",
+        sender="Me <me@example.com>",
+    )
+    remote = RemoteMessage(uid=9, raw=raw, flags=frozenset(), internal_date=datetime.now(UTC))
+    await persist_message(
+        api.session,
+        user_id=api.user_id,
+        account_id=api.account_id,
+        mailbox_id=inbox.id,
+        mailbox_special_use="inbox",
+        remote=remote,
+    )
+    await api.session.commit()
+
+    resp = await api.client.get("/api/threads")
+    kickoff = next(t for t in resp.json()["threads"] if "kickoff" in t["subject"].lower())
+    assert [a["email"] for a in kickoff["senders"]] == ["alice@example.com", "me@example.com"]
+    lunch = next(t for t in resp.json()["threads"] if t["subject"] == "Lunch")
+    assert [a["email"] for a in lunch["senders"]] == ["alice@example.com"]
+
+
 async def test_folder_view(api: ApiCtx) -> None:
     # all seeded mail is in an inbox mailbox
     inbox = await api.client.get("/api/threads", params={"view": "folder:inbox"})