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.

scripts/db/init_postgres.py

@@ -13,7 +13,7 @@ Intended use:
 
 This script is a thin wrapper around ``alembic upgrade head``. It exists
 separately so the same command is discoverable from the repo root without
-remembering the ``-c application/alembic.ini`` invocation.
+remembering the ``-c docsgpt/alembic.ini`` invocation.
 """
 
 from __future__ import annotations
@@ -25,7 +25,7 @@ from alembic import command
 from alembic.config import Config
 
 REPO_ROOT = Path(__file__).resolve().parents[2]
-ALEMBIC_INI = REPO_ROOT / "application" / "alembic.ini"
+ALEMBIC_INI = REPO_ROOT / "docsgpt" / "alembic.ini"
 
 
 def main() -> int: