refactor: rename the application package to docsgpt

The backend import package is now docsgpt, the name it will carry on PyPI;
application was far too generic to install into anyone's site-packages.
git mv plus a mechanical rewrite of every import, dotted string and path
reference: 734 Python files, the compose files, Dockerfile, workflows, docs,
setup scripts, devcontainer, k8s manifests, vscode config, pytest and coverage
config, .gitignore. Behaviour is unchanged.

Kept for one release:
- A top-level application package whose meta-path finder resolves
  application.x.y to the already-imported docsgpt.x.y object, so old imports
  and entry points (celery -A application.app.celery,
  uvicorn application.asgi:asgi_app) keep working with a FutureWarning.
- Celery registers every application.* task name as an alias of its
  docsgpt.* task on start-up, so messages queued by the previous release still
  run. The redbeat key prefix moves to redbeat:docsgpt:v2: so schedule entries
  the previous release wrote are left unread instead of firing twice.

The backend image builds from the repository root (docker build -f
docsgpt/Dockerfile .) so it can ship the alias package; a root .dockerignore
allow-lists docsgpt/ and application/ and keeps caches, local data, .env
files, the sample index files and the Dockerfile out. Compose and the image
workflows point at the new context.

tests/integration/test_workflows.py

@@ -27,7 +27,7 @@ from jose import jwt
 @pytest.fixture(scope="module")
 def app():
     """Create the real Flask app (connects to real Postgres)."""
-    from application.app import app as flask_app
+    from docsgpt.app import app as flask_app
     flask_app.config["TESTING"] = True
     return flask_app
 
@@ -40,7 +40,7 @@ def client(app):
     injected; otherwise the backend already returns {"sub": "local"}
     for every request so no token is needed.
     """
-    from application.core.settings import settings
+    from docsgpt.core.settings import settings
 
     c = app.test_client()
     if settings.AUTH_TYPE in ("simple_jwt", "session_jwt"):