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.

docs/content/Deploying/Docker-Deploying.mdx

@@ -69,8 +69,8 @@ checkout.
 ## Using the Source Checkout
 
 With a clone of the repository, `deployment/docker-compose-hub.yaml` runs the
-same pre-built images while keeping your data in `application/indexes`,
-`application/inputs` and `application/vectors`, and `deployment/docker-compose.yaml`
+same pre-built images while keeping your data in `docsgpt/indexes`,
+`docsgpt/inputs` and `docsgpt/vectors`, and `deployment/docker-compose.yaml`
 builds the images from your working tree (for local changes, or a build with
 extra packages: `EXTRAS=docling` in `.env`).