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_audio_parser.py

@@ -1,8 +1,8 @@
 from unittest.mock import MagicMock, patch
 
-from application.parser.file.audio_parser import AudioParser
-from application.parser.file.bulk import get_default_file_extractor
-from application.stt.upload_limits import AudioFileTooLargeError
+from docsgpt.parser.file.audio_parser import AudioParser
+from docsgpt.parser.file.bulk import get_default_file_extractor
+from docsgpt.stt.upload_limits import AudioFileTooLargeError
 
 
 def test_audio_init_parser():
@@ -13,9 +13,9 @@ def test_audio_init_parser():
     assert parser.parser_config_set
 
 
-@patch("application.stt.upload_limits.settings")
-@patch("application.parser.file.audio_parser.STTCreator.create_stt")
-@patch("application.parser.file.audio_parser.settings")
+@patch("docsgpt.stt.upload_limits.settings")
+@patch("docsgpt.parser.file.audio_parser.STTCreator.create_stt")
+@patch("docsgpt.parser.file.audio_parser.settings")
 def test_audio_parser_transcribes_file(
     mock_settings, mock_create_stt, mock_limit_settings, tmp_path
 ):
@@ -44,7 +44,7 @@ def test_audio_parser_transcribes_file(
     )
 
 
-@patch("application.stt.upload_limits.settings")
+@patch("docsgpt.stt.upload_limits.settings")
 def test_audio_parser_rejects_oversized_files(mock_limit_settings, tmp_path):
     mock_limit_settings.STT_MAX_FILE_SIZE_MB = 1