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.

.vscode/launch.json

@@ -14,7 +14,7 @@
         "request": "launch",
         "module": "flask",
         "env": {
-            "FLASK_APP": "application/app.py",
+            "FLASK_APP": "docsgpt/app.py",
             "PYTHONPATH": "${workspaceFolder}",
             "FLASK_ENV": "development",
             "FLASK_DEBUG": "1",
@@ -38,7 +38,7 @@
       },
       "args": [
         "-A",
-        "application.app.celery",
+        "docsgpt.app.celery",
         "worker",
         "-l",
         "INFO",