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_tool_executor_headless.py

@@ -4,7 +4,7 @@ from __future__ import annotations
 
 from types import SimpleNamespace
 
-from application.agents.tool_executor import ToolExecutor
+from docsgpt.agents.tool_executor import ToolExecutor
 
 
 def _call(name: str, args: dict | None = None, call_id: str = "c1"):
@@ -97,7 +97,7 @@ class TestNormalModeUnchanged:
 # ---------------------------------------------------------------------------
 class TestHeadlessSchedulerExclusion:
     def test_synthesized_default_tools_drops_scheduler_in_headless(self):
-        from application.agents.default_tools import (
+        from docsgpt.agents.default_tools import (
             loaded_default_tools,
             synthesized_default_tools,
         )
@@ -116,8 +116,8 @@ class TestHeadlessSchedulerExclusion:
     def test_get_user_tools_filters_scheduler_when_headless(
         self, monkeypatch,
     ):
-        from application.agents import tool_executor as te_module
-        from application.agents.default_tools import (
+        from docsgpt.agents import tool_executor as te_module
+        from docsgpt.agents.default_tools import (
             default_tool_id,
             loaded_default_tools,
         )
@@ -164,8 +164,8 @@ class TestHeadlessSchedulerExclusion:
     ):
         """An agent-bound headless run (e.g. webhook) skips scheduler even if
         the author added the synthetic id to ``agents.tools``."""
-        from application.agents import tool_executor as te_module
-        from application.agents.default_tools import default_tool_id
+        from docsgpt.agents import tool_executor as te_module
+        from docsgpt.agents.default_tools import default_tool_id
 
         sched_id = default_tool_id("scheduler")
         from contextlib import contextmanager