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_pdfium_parser.py

@@ -13,7 +13,7 @@ the fast path must only appear when a caller explicitly asks for it.
 
 import pytest
 
-from application.parser.file.base_parser import BaseParser, DocumentParseError
+from docsgpt.parser.file.base_parser import BaseParser, DocumentParseError
 
 pypdfium2 = pytest.importorskip("pypdfium2")
 
@@ -66,7 +66,7 @@ class _RecordingFallback(BaseParser):
 
 @pytest.fixture
 def parser_cls():
-    from application.parser.file.pdfium_parser import PdfiumTextParser
+    from docsgpt.parser.file.pdfium_parser import PdfiumTextParser
 
     return PdfiumTextParser
 
@@ -162,7 +162,7 @@ def docling_engine(monkeypatch):
     # (docling is an optional extra) the engine degrades to the legacy
     # parsers and these tests do not apply.
     pytest.importorskip("docling")
-    from application.core.settings import settings
+    from docsgpt.core.settings import settings
 
     monkeypatch.setattr(settings, "DOC_PARSER_ENGINE", "docling")
     # A developer .env with OCR on (and OCR_BACKEND=native) would otherwise
@@ -172,7 +172,7 @@ def docling_engine(monkeypatch):
 
 def test_extractor_defaults_to_docling_for_pdf(docling_engine):
     """Sources must be unaffected: no fast path unless explicitly requested."""
-    from application.parser.file.bulk import get_default_file_extractor
+    from docsgpt.parser.file.bulk import get_default_file_extractor
 
     pdf_parser = get_default_file_extractor()[".pdf"]
 
@@ -180,8 +180,8 @@ def test_extractor_defaults_to_docling_for_pdf(docling_engine):
 
 
 def test_extractor_uses_fast_path_when_requested(docling_engine):
-    from application.parser.file.bulk import get_default_file_extractor
-    from application.parser.file.pdfium_parser import PdfiumTextParser
+    from docsgpt.parser.file.bulk import get_default_file_extractor
+    from docsgpt.parser.file.pdfium_parser import PdfiumTextParser
 
     pdf_parser = get_default_file_extractor(pdf_text_fast_path=True)[".pdf"]
 
@@ -192,8 +192,8 @@ def test_extractor_uses_fast_path_when_requested(docling_engine):
 def test_anydoc_engine_ignores_fast_path(monkeypatch):
     """anydoc already reads the text layer in milliseconds and keeps structure."""
     pytest.importorskip("anydoc")
-    from application.core.settings import settings
-    from application.parser.file.bulk import get_default_file_extractor
+    from docsgpt.core.settings import settings
+    from docsgpt.parser.file.bulk import get_default_file_extractor
 
     monkeypatch.setattr(settings, "DOC_PARSER_ENGINE", "anydoc")
 
@@ -203,7 +203,7 @@ def test_anydoc_engine_ignores_fast_path(monkeypatch):
 
 
 def test_fast_path_does_not_change_non_pdf_parsers(docling_engine):
-    from application.parser.file.bulk import get_default_file_extractor
+    from docsgpt.parser.file.bulk import get_default_file_extractor
 
     plain = get_default_file_extractor()
     fast = get_default_file_extractor(pdf_text_fast_path=True)