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_node_agent.py

@@ -13,14 +13,14 @@ def _no_db(monkeypatch):
     def fake_readonly():
         yield MagicMock()
 
-    monkeypatch.setattr("application.agents.tool_executor.db_readonly", fake_readonly)
+    monkeypatch.setattr("docsgpt.agents.tool_executor.db_readonly", fake_readonly)
 
 
 class _FakeExecutorBase:
     """Stands in for BaseAgent: provides the tool_executor the mixin scopes."""
 
     def __init__(self, *args, **kwargs):
-        from application.agents.tool_executor import ToolExecutor
+        from docsgpt.agents.tool_executor import ToolExecutor
 
         self.tool_executor = ToolExecutor()
 
@@ -29,7 +29,7 @@ class _FakeExecutorBase:
 class TestWorkflowNodeAgentFactory:
 
     def test_raises_on_unsupported_type(self):
-        from application.agents.workflows.node_agent import WorkflowNodeAgentFactory
+        from docsgpt.agents.workflows.node_agent import WorkflowNodeAgentFactory
 
         with pytest.raises(ValueError, match="Unsupported agent type"):
             WorkflowNodeAgentFactory.create(
@@ -47,7 +47,7 @@ class TestWorkflowNodeMixinInit:
     so per-node tool filtering must live there (the old agent-method mixin was dead code)."""
 
     def _mixed(self, **kwargs):
-        from application.agents.workflows.node_agent import _WorkflowNodeMixin
+        from docsgpt.agents.workflows.node_agent import _WorkflowNodeMixin
 
         class TestMixin(_WorkflowNodeMixin, _FakeExecutorBase):
             pass
@@ -76,7 +76,7 @@ class TestWorkflowNodeMixinInit:
     def test_scoped_executor_resolves_builtin_synthetic_ids(self, _no_db):
         """A node whose Tools picker selected a builtin (Artifact / Read Document)
         must get that tool at runtime — the P0 this design replaced."""
-        from application.agents.default_tools import default_tool_id
+        from docsgpt.agents.default_tools import default_tool_id
 
         obj = self._mixed(
             tool_ids=[