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/core/test_model_settings.py

@@ -1,9 +1,9 @@
-"""Tests for application/core/model_settings.py.
+"""Tests for docsgpt/core/model_settings.py.
 
 The provider-specific load logic that used to live in private
 ``_add_<X>_models`` methods now lives in plugin classes under
-``application/llm/providers/`` and YAML catalogs under
-``application/core/models/``. End-to-end coverage of the registry +
+``docsgpt/llm/providers/`` and YAML catalogs under
+``docsgpt/core/models/``. End-to-end coverage of the registry +
 plugin pipeline is in ``tests/core/test_model_registry_yaml.py``.
 
 This file covers the data classes (``AvailableModel``,
@@ -15,7 +15,7 @@ from unittest.mock import patch
 
 import pytest
 
-from application.core.model_settings import (
+from docsgpt.core.model_settings import (
     AvailableModel,
     ModelCapabilities,
     ModelProvider,