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/README.md

@@ -1,4 +1,4 @@
-# KeepItTalking backend (Phase 1 + 2 + 4 + 5 + 6 + 7 + 8, image uploads, file attachments, admin-configurable upload size limits, emoji & reactions, user profiles, site invites & email, password reset)
+# DS Chat backend (Phase 1 + 2 + 4 + 5 + 6 + 7 + 8, image uploads, file attachments, admin-configurable upload size limits, emoji & reactions, user profiles, site invites & email, password reset)
 
 FastAPI + SQLAlchemy 2.0 (async) + PostgreSQL + Redis. Implements auth, room
 CRUD (open and private), room roles (owner/admin/member) and direct
@@ -24,15 +24,15 @@ Accounts are created by an operator on the app server — see step 4 below.
 Any local Postgres 14+ works. The quickest option is a container:
 
 ```bash
-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
 ```
 
 Then create the test database (used by the test suite, kept separate from dev data):
 
 ```bash
-docker exec chatapp-postgres psql -U chatapp -d chatapp -c "CREATE DATABASE chatapp_test;"
+docker exec ds-chat-postgres psql -U ds_chat -d ds_chat -c "CREATE DATABASE ds_chat_test;"
 ```
 
 (Docker here is purely a local-dev convenience for standing up Postgres quickly —
@@ -44,7 +44,7 @@ Used for cross-instance WebSocket fan-out and presence (see the section
 below). Required — there's no in-memory fallback.
 
 ```bash
-docker run -d --name chatapp-redis -p 6379:6379 redis:7-alpine
+docker run -d --name ds-chat-redis -p 6379:6379 redis:7-alpine
 ```
 
 ### 3. Python environment
@@ -99,13 +99,13 @@ connected to the other, purely via Redis.
 
 ### 8. Run tests
 
-Tests run against a real Postgres database (`chatapp_test` by default — native
+Tests run against a real Postgres database (`ds_chat_test` by default — native
 `ENUM`/`UUID` types aren't faithfully reproduced by SQLite) and a real Redis
 (db 15 by default, kept separate from dev use of db 0), with each test
 wrapped in a transaction that's rolled back afterward:
 
 ```bash
-DATABASE_URL=postgresql+asyncpg://chatapp:chatapp@localhost:5432/chatapp_test .venv/bin/pytest
+DATABASE_URL=postgresql+asyncpg://ds_chat:ds_chat@localhost:5432/ds_chat_test .venv/bin/pytest
 ```
 
 ## Layout
@@ -247,7 +247,7 @@ call sites rather than duplicated).
 **Outgoing webhooks / event subscriptions** (`POST
 /api/rooms/{id}/event-subscriptions`, room-admin managed; room-scoped or
 global via `room_id=null`): fires an HMAC-SHA256-signed POST
-(`X-KeepItTalking-Signature: sha256=...`) on `message.created`/
+(`X-DS-Chat-Signature: sha256=...`) on `message.created`/
 `message.updated`, delivered via a backgrounded `asyncio.create_task`
 (`app/services/webhook_delivery.py`) — safe to background here, unlike the
 Phase 4 push lesson, since there's no DB session involved, just the