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/api/user/test_models.py

@@ -14,7 +14,7 @@ def app():
 class TestModelsListResource:
 
     def test_returns_models(self, app):
-        from application.api.user.models.routes import ModelsListResource
+        from docsgpt.api.user.models.routes import ModelsListResource
 
         mock_model = Mock()
         mock_model.to_dict.return_value = {
@@ -28,7 +28,7 @@ class TestModelsListResource:
         mock_registry.default_model_id = "gpt-4"
 
         with patch(
-            "application.api.user.models.routes.ModelRegistry.get_instance",
+            "docsgpt.api.user.models.routes.ModelRegistry.get_instance",
             return_value=mock_registry,
         ):
             with app.test_request_context("/api/models"):
@@ -40,14 +40,14 @@ class TestModelsListResource:
         assert response.json["models"][0]["id"] == "gpt-4"
 
     def test_returns_empty_models(self, app):
-        from application.api.user.models.routes import ModelsListResource
+        from docsgpt.api.user.models.routes import ModelsListResource
 
         mock_registry = Mock()
         mock_registry.get_enabled_models.return_value = []
         mock_registry.default_model_id = None
 
         with patch(
-            "application.api.user.models.routes.ModelRegistry.get_instance",
+            "docsgpt.api.user.models.routes.ModelRegistry.get_instance",
             return_value=mock_registry,
         ):
             with app.test_request_context("/api/models"):
@@ -58,10 +58,10 @@ class TestModelsListResource:
         assert response.json["models"] == []
 
     def test_returns_500_on_error(self, app):
-        from application.api.user.models.routes import ModelsListResource
+        from docsgpt.api.user.models.routes import ModelsListResource
 
         with patch(
-            "application.api.user.models.routes.ModelRegistry.get_instance",
+            "docsgpt.api.user.models.routes.ModelRegistry.get_instance",
             side_effect=Exception("Registry error"),
         ):
             with app.test_request_context("/api/models"):