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_workflow_agent_pg_write.py

@@ -39,8 +39,8 @@ def _make_agent(**overrides):
         "json_schema": None,
     }
     defaults.update(overrides)
-    with patch("application.agents.workflow_agent.log_activity", lambda **kw: lambda f: f):
-        from application.agents.workflow_agent import WorkflowAgent
+    with patch("docsgpt.agents.workflow_agent.log_activity", lambda **kw: lambda f: f):
+        from docsgpt.agents.workflow_agent import WorkflowAgent
 
         agent = WorkflowAgent(**defaults)
     return agent
@@ -114,7 +114,7 @@ class TestDetermineRunStatusPg:
 
     def test_completed_value_matches_pg_enum(self):
         """The string stored in Postgres must match ExecutionStatus.COMPLETED.value."""
-        from application.agents.workflows.schemas import ExecutionStatus
+        from docsgpt.agents.workflows.schemas import ExecutionStatus
 
         agent = _make_agent()
         agent._engine = MagicMock()
@@ -125,7 +125,7 @@ class TestDetermineRunStatusPg:
         assert status.value == "completed"
 
     def test_failed_value_matches_pg_enum(self):
-        from application.agents.workflows.schemas import ExecutionStatus
+        from docsgpt.agents.workflows.schemas import ExecutionStatus
 
         agent = _make_agent()
         agent._engine = MagicMock()