Add an auto-provisioned system account for automated messages (#74)

The #72 welcome message was attributed to whichever admin added the
member, since there was no system/bot sender concept -- every Message
row requires a real user_id. Adds a lazily-created "system" bot
account (reusing the existing is_bot infrastructure) and switches the
welcome message to post as it instead. Excluded from the People
directory and @mention autocomplete for free: the directory already
filters is_bot users, and mention suggestions are sourced from room
membership, which the system account is never added to.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/tests/test_rooms.py

@@ -485,10 +485,10 @@ async def test_add_member_directly(client, db_session, monkeypatch):
 
 
 async def test_add_member_posts_welcome_message(client, db_session, monkeypatch):
-    # #72: attributed to the admin who added them, since there's no
-    # system/bot sender concept -- mirrors test_add_member_directly's setup.
+    # #74: posted as the auto-provisioned "system" account, not the admin
+    # who added them -- mirrors test_add_member_directly's setup.
     _fake_send_email(monkeypatch)
-    alice = await register_and_login(client, db_session, username="alice")
+    await register_and_login(client, db_session, username="alice")
     room_id = (await client.post("/api/rooms", json={"name": "general"})).json()["id"]
 
     await client.post("/api/auth/logout")
@@ -500,7 +500,7 @@ async def test_add_member_posts_welcome_message(client, db_session, monkeypatch)
     assert resp.status_code == 201, resp.text
 
     history = (await client.get(f"/api/rooms/{room_id}/messages")).json()
-    welcome_messages = [m for m in history if m["username"] == "alice"]
+    welcome_messages = [m for m in history if m["username"] == "system"]
     assert len(welcome_messages) == 1
     assert welcome_messages[0]["content"] == "Welcome to #general, bob!"
 

backend/tests/test_system_user.py

@@ -0,0 +1,13 @@
+from app.services.system_user_service import SYSTEM_USERNAME, get_or_create_system_user
+
+
+async def test_get_or_create_system_user_creates_bot_account(db_session):
+    user = await get_or_create_system_user(db_session)
+    assert user.username == SYSTEM_USERNAME
+    assert user.is_bot is True
+
+
+async def test_get_or_create_system_user_is_idempotent(db_session):
+    first = await get_or_create_system_user(db_session)
+    second = await get_or_create_system_user(db_session)
+    assert first.id == second.id