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.

scripts/e2e/mock_llm.py

@@ -28,7 +28,7 @@ verbatim as the conversation's prompt). See
 
 Run standalone (does NOT import anything from ``application/``). Python 3.11+.
 Flask is the only non-stdlib dependency and is already in
-``application/requirements.txt``.
+``docsgpt/requirements.txt``.
 
 Usage::
 
@@ -482,7 +482,7 @@ def _stream_chat_response(
     if tool_calls and toolcall_arg_mode:
         # Frame-split mode: the call's ``arguments`` are spread over several
         # deltas that all share one ``index``, which is what the client-side
-        # merge in application/llm/handlers/base.py reassembles.
+        # merge in docsgpt/llm/handlers/base.py reassembles.
         call = tool_calls[0]
         frames = _toolcall_arg_frames(call["function"]["arguments"], toolcall_arg_mode)
         for position, piece in enumerate(frames):