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/agents/test_headless_runner_errors.py

@@ -18,7 +18,7 @@ import pytest
 
 def _run(events, monkeypatch):
     """Drive ``run_agent_headless`` over a canned agent event stream."""
-    from application.agents import headless_runner as hr
+    from docsgpt.agents import headless_runner as hr
 
     agent = MagicMock(name="agent")
     agent.gen.return_value = iter(events)
@@ -41,14 +41,14 @@ def _run(events, monkeypatch):
         classmethod(lambda cls, *a, **kw: agent),
     )
 
-    with patch("application.core.model_utils.validate_model_id", return_value=True), \
-         patch("application.core.model_utils.get_default_model_id", return_value="m"), \
+    with patch("docsgpt.core.model_utils.validate_model_id", return_value=True), \
+         patch("docsgpt.core.model_utils.get_default_model_id", return_value="m"), \
          patch(
-             "application.core.model_utils.get_provider_from_model_id",
+             "docsgpt.core.model_utils.get_provider_from_model_id",
              return_value="openai",
          ), \
-         patch("application.core.model_utils.get_api_key_for_provider", return_value="k"), \
-         patch("application.utils.calculate_doc_token_budget", return_value=1000):
+         patch("docsgpt.core.model_utils.get_api_key_for_provider", return_value="k"), \
+         patch("docsgpt.utils.calculate_doc_token_budget", return_value=1000):
         return hr.run_agent_headless(
             {"user_id": "u1", "id": "agent-1", "default_model_id": "m"},
             "do the thing",