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.

.github/workflows/backend-release.yml

@@ -4,7 +4,7 @@ on:
   push:
     branches: [main]
     paths:
-      - 'application/version.py'
+      - 'docsgpt/version.py'
   workflow_dispatch:
 
 permissions:
@@ -23,12 +23,12 @@ jobs:
         with:
           persist-credentials: false
 
-      - name: Read version from application/version.py
+      - name: Read version from docsgpt/version.py
         id: ver
         run: |
-          VERSION=$(python3 -c "g={}; exec(open('application/version.py').read(), g); print(g['__version__'])")
+          VERSION=$(python3 -c "g={}; exec(open('docsgpt/version.py').read(), g); print(g['__version__'])")
           if [ -z "$VERSION" ]; then
-            echo "::error::Could not read __version__ from application/version.py"
+            echo "::error::Could not read __version__ from docsgpt/version.py"
             exit 1
           fi
           echo "version=$VERSION" >> "$GITHUB_OUTPUT"