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/test_app.py

@@ -1,8 +1,8 @@
 import pytest
-from application.api.answer import answer
-from application.api.internal.routes import internal
-from application.api.user.routes import user
-from application.core.settings import settings
+from docsgpt.api.answer import answer
+from docsgpt.api.internal.routes import internal
+from docsgpt.api.user.routes import user
+from docsgpt.core.settings import settings
 from flask import Flask
 
 
@@ -30,8 +30,8 @@ class TestLogContextTeardown:
     def test_reset_is_idempotent(self):
         from flask import request
 
-        from application.app import _LOG_CTX_TOKEN_ATTR, _reset_log_context, app
-        from application.core import log_context
+        from docsgpt.app import _LOG_CTX_TOKEN_ATTR, _reset_log_context, app
+        from docsgpt.core import log_context
 
         with app.test_request_context("/"):
             token = log_context.bind(activity_id="abc", endpoint="stream")