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>

README.md

@@ -1,4 +1,4 @@
-# KeepItTalking
+# DS Chat
 
 A web-based team chat service (Mattermost-style, no threaded conversations),
 invite-only. See [ARCHITECTURE.md](ARCHITECTURE.md) for the full system design
@@ -25,8 +25,8 @@ prioritized.
 
 ```bash
 # 1. Postgres (see backend/README.md for details)
-docker run -d --name chatapp-postgres \
-  -e POSTGRES_USER=chatapp -e POSTGRES_PASSWORD=chatapp -e POSTGRES_DB=chatapp \
+docker run -d --name ds-chat-postgres \
+  -e POSTGRES_USER=ds_chat -e POSTGRES_PASSWORD=ds_chat -e POSTGRES_DB=ds_chat \
   -p 5432:5432 postgres:16-alpine
 
 # 2. Backend