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/scripts/test_verify_offline.py

@@ -4,7 +4,7 @@ import sys
 import types
 from unittest.mock import patch
 
-from application.scripts import verify_offline
+from docsgpt.scripts import verify_offline
 
 
 def _fake_tiktoken(monkeypatch):
@@ -17,8 +17,8 @@ class TestVerify:
     def test_passes_when_every_check_passes(self, monkeypatch, capsys):
         _fake_tiktoken(monkeypatch)
         counter = types.SimpleNamespace(name="org/model", count=lambda text: 4)
-        with patch("application.parser.tokenization.get_token_counter", return_value=counter), \
-                patch("application.vectorstore.embeddings_local.EmbeddingsWrapper") as wrapper, \
+        with patch("docsgpt.parser.tokenization.get_token_counter", return_value=counter), \
+                patch("docsgpt.vectorstore.embeddings_local.EmbeddingsWrapper") as wrapper, \
                 patch.object(verify_offline, "is_available", return_value=False):
             wrapper.return_value.embed_query.return_value = [0.0] * 768
             assert verify_offline.verify(["ibm-granite/granite-embedding-311m-multilingual-r2"]) is True
@@ -30,8 +30,8 @@ class TestVerify:
         """A cache miss makes chunking silently use cl100k; that is a failed check."""
         _fake_tiktoken(monkeypatch)
         counter = types.SimpleNamespace(name="cl100k_base", count=lambda text: 4)
-        with patch("application.parser.tokenization.get_token_counter", return_value=counter), \
-                patch("application.vectorstore.embeddings_local.EmbeddingsWrapper") as wrapper, \
+        with patch("docsgpt.parser.tokenization.get_token_counter", return_value=counter), \
+                patch("docsgpt.vectorstore.embeddings_local.EmbeddingsWrapper") as wrapper, \
                 patch.object(verify_offline, "is_available", return_value=False):
             wrapper.return_value.embed_query.return_value = [0.0] * 768
             assert verify_offline.verify(["ibm-granite/granite-embedding-311m-multilingual-r2"]) is False