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/agents/test_cel_evaluator.py

@@ -1,8 +1,8 @@
-"""Tests for application/agents/workflows/cel_evaluator.py"""
+"""Tests for docsgpt/agents/workflows/cel_evaluator.py"""
 
 import pytest
 
-from application.agents.workflows.cel_evaluator import (
+from docsgpt.agents.workflows.cel_evaluator import (
     CelEvaluationError,
     _convert_value,
     build_activation,
@@ -232,7 +232,7 @@ class TestCelErrorMessages:
     @pytest.mark.unit
     def test_long_error_is_truncated(self):
         """Backstop for celpy messages that survive activation-stripping."""
-        from application.agents.workflows.cel_evaluator import _summarize_cel_error
+        from docsgpt.agents.workflows.cel_evaluator import _summarize_cel_error
 
         summary = _summarize_cel_error(Exception("word " * 200))
         assert len(summary) <= 200