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/parser/file/test_image_parser.py

@@ -1,7 +1,7 @@
 from pathlib import Path
 from unittest.mock import patch, MagicMock, mock_open
 
-from application.parser.file.image_parser import ImageParser
+from docsgpt.parser.file.image_parser import ImageParser
 
 
 def test_image_init_parser():
@@ -12,7 +12,7 @@ def test_image_init_parser():
     assert parser.parser_config_set
 
 
-@patch("application.parser.file.image_parser.settings")
+@patch("docsgpt.parser.file.image_parser.settings")
 def test_image_parser_remote_true(mock_settings):
     mock_settings.PARSE_IMAGE_REMOTE = True
     parser = ImageParser()
@@ -20,7 +20,7 @@ def test_image_parser_remote_true(mock_settings):
     mock_response = MagicMock()
     mock_response.json.return_value = {"markdown": "# From Image"}
 
-    with patch("application.parser.file.image_parser.requests.post", return_value=mock_response) as mock_post:
+    with patch("docsgpt.parser.file.image_parser.requests.post", return_value=mock_response) as mock_post:
         with patch("builtins.open", mock_open()):
             result = parser.parse_file(Path("img.png"))
 
@@ -28,12 +28,12 @@ def test_image_parser_remote_true(mock_settings):
     mock_post.assert_called_once()
 
 
-@patch("application.parser.file.image_parser.settings")
+@patch("docsgpt.parser.file.image_parser.settings")
 def test_image_parser_remote_false(mock_settings):
     mock_settings.PARSE_IMAGE_REMOTE = False
     parser = ImageParser()
 
-    with patch("application.parser.file.image_parser.requests.post") as mock_post:
+    with patch("docsgpt.parser.file.image_parser.requests.post") as mock_post:
         result = parser.parse_file(Path("img.png"))
 
     assert result == ""