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/tests/conftest.py

@@ -3,7 +3,7 @@ import os
 from pathlib import Path
 
 os.environ.setdefault(
-    "DATABASE_URL", "postgresql+asyncpg://chatapp:chatapp@localhost:5432/chatapp_test"
+    "DATABASE_URL", "postgresql+asyncpg://chatapp:chatapp@localhost:5432/ds_chat_test"
 )
 os.environ.setdefault("SESSION_SECRET", "test-secret")
 os.environ.setdefault("SESSION_HTTPS_ONLY", "false")

backend/tests/test_push.py

@@ -31,7 +31,7 @@ async def test_subscribe_creates_row(client, db_session):
     resp = await client.post("/api/push/subscribe", json=payload)
     assert resp.status_code == 204
 
-    # The chatapp_test database is shared across the whole suite and the
+    # The ds_chat_test database is shared across the whole suite and the
     # ws_client-based tests below intentionally don't roll back (see
     # conftest.ws_client), so a unique endpoint keeps this test independent
     # of leftover rows from those instead of asserting on the total count.

backend/tests/test_webhooks.py

@@ -158,7 +158,7 @@ async def test_outgoing_webhook_delivers_signed_payload(client, db_session, monk
     assert len(posts) == 1
     body = posts[0]["content"]
     expected_signature = hmac.new(secret.encode(), body, hashlib.sha256).hexdigest()
-    assert posts[0]["headers"]["X-KeepItTalking-Signature"] == f"sha256={expected_signature}"
+    assert posts[0]["headers"]["X-DS-Chat-Signature"] == f"sha256={expected_signature}"
     payload = json.loads(body)
     assert payload["event"] == "message.created"
     assert payload["data"]["content"] == "ping"