Rename project from KeepItTalking to DS Chat

Renames the app's display name everywhere (page titles, PWA manifest,
TopBar, email subject lines, HMAC signature header) and its internal
technical slug from chatapp to ds-chat/ds_chat: the Python package name
and console script, the systemd unit and its user/group/paths, the deploy
scripts, the Docker container names, and the Postgres database name.

The live dev Postgres role stays "chatapp" -- renaming a role requires
disconnecting the session using it, which needed a temporary superuser
role Claude's auto-mode classifier correctly declined to create
unsupervised. Functionally invisible (it's just a login credential), but
worth knowing about if this ever needs fully cleaning up by hand.

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

backend/app/services/password_service.py

@@ -44,7 +44,7 @@ async def request_password_reset(db: AsyncSession, email: str, base_url: str) ->
     await send_email(
         db,
         email,
-        "Reset your KeepItTalking password",
+        "Reset your DS Chat password",
         f"Someone requested a password reset for this account.\n\n"
         f"Reset it here:\n{reset_link}\n\n"
         f"This link expires in 15 minutes. If you didn't request this, "