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/test_zip_extraction_security.py

@@ -7,14 +7,14 @@ import zipfile
 
 import pytest
 
-from application.worker import (
+from docsgpt.worker import (
     ZipExtractionError,
     _is_path_safe,
     _validate_zip_safety,
     extract_zip_recursive,
     MAX_FILE_COUNT,
 )
-from application.security.zip_archive import (
+from docsgpt.security.zip_archive import (
     extract_zip_safely,
     ZipExtractionLimits,
 )
@@ -34,8 +34,8 @@ def _test_limits(**overrides):
 
 class TestTransactionalZipExtraction:
     def test_default_file_budget_accepts_large_source_archives(self, tmp_path):
-        from application.api.user.sources.upload import _source_archive_limits
-        from application.security.zip_archive import validate_zip_archive
+        from docsgpt.api.user.sources.upload import _source_archive_limits
+        from docsgpt.security.zip_archive import validate_zip_archive
 
         zip_path = tmp_path / "repository.zip"
         with zipfile.ZipFile(zip_path, "w") as archive: