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/answer/test_stream_processor_exposure.py

@@ -6,8 +6,8 @@ from unittest.mock import MagicMock, patch
 
 import pytest
 
-from application.api.answer.services.stream_processor import StreamProcessor
-from application.storage.db.source_config import RetrievalConfig
+from docsgpt.api.answer.services.stream_processor import StreamProcessor
+from docsgpt.storage.db.source_config import RetrievalConfig
 
 
 def _processor() -> StreamProcessor:
@@ -164,12 +164,12 @@ class TestNonAgentSourceConfig:
             "config": {"retrieval": {"exposure": "agentic_tool", "chunks": 7}}
         }
         with patch(
-            "application.api.answer.services.stream_processor.db_readonly"
+            "docsgpt.api.answer.services.stream_processor.db_readonly"
         ), patch(
-            "application.api.answer.services.stream_processor.can_access",
+            "docsgpt.api.answer.services.stream_processor.can_access",
             return_value=True,
         ), patch(
-            "application.api.answer.services.stream_processor.SourcesRepository"
+            "docsgpt.api.answer.services.stream_processor.SourcesRepository"
         ) as repo:
             # Read unscoped after the access check, so a team grantee gets the
             # source's real config instead of silently falling back to defaults.