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/tools/test_notes_pg.py

@@ -1,4 +1,4 @@
-"""Tests for application/agents/tools/notes.py using pg_conn."""
+"""Tests for docsgpt/agents/tools/notes.py using pg_conn."""
 
 from contextlib import contextmanager
 from unittest.mock import patch
@@ -12,15 +12,15 @@ def _patch_db(conn):
         yield conn
 
     with patch(
-        "application.storage.db.session.db_readonly", _yield
+        "docsgpt.storage.db.session.db_readonly", _yield
     ), patch(
-        "application.storage.db.session.db_session", _yield
+        "docsgpt.storage.db.session.db_session", _yield
     ):
         yield
 
 
 def _make_tool(tool_id="default_test", user_id="u"):
-    from application.agents.tools.notes import NotesTool
+    from docsgpt.agents.tools.notes import NotesTool
     tool = NotesTool.__new__(NotesTool)
     tool.tool_id = tool_id
     tool.user_id = user_id
@@ -60,7 +60,7 @@ class TestNotesToolExecuteGuards:
 
     def test_unknown_action(self, pg_conn):
         # Real tool_id requires a user_tools row
-        from application.storage.db.repositories.user_tools import (
+        from docsgpt.storage.db.repositories.user_tools import (
             UserToolsRepository,
         )
         UserToolsRepository(pg_conn).create("u", "notes_tool")