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_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