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.

docs/content/Deploying/Postgres-Migration.mdx

@@ -37,7 +37,7 @@ schema on first boot.
 
 ```bash
 export POSTGRES_URI="postgresql://user:pass@host/docsgpt?sslmode=require"
-uvicorn application.asgi:asgi_app --host 0.0.0.0 --port 7091
+uvicorn docsgpt.asgi:asgi_app --host 0.0.0.0 --port 7091
 ```
 
 ### Bare-metal Postgres
@@ -47,7 +47,7 @@ First boot creates both the database and the schema.
 
 ```bash
 export POSTGRES_URI="postgresql://postgres@localhost/docsgpt"
-uvicorn application.asgi:asgi_app --host 0.0.0.0 --port 7091
+uvicorn docsgpt.asgi:asgi_app --host 0.0.0.0 --port 7091
 ```
 
 Prefer a dedicated non-superuser role? Create it once as superuser — the
@@ -93,7 +93,7 @@ init-container ahead of the app rollout:
 ```bash
 python scripts/db/init_postgres.py
 # equivalently:
-alembic -c application/alembic.ini upgrade head
+alembic -c docsgpt/alembic.ini upgrade head
 ```
 
 The reasoning: the app's runtime role shouldn't carry DDL privileges,
@@ -112,7 +112,7 @@ One-shot, offline, app stopped. The app itself will create the
 Postgres schema when it boots — you only need to run the data copy.
 
 ```bash
-pip install -r application/requirements.txt
+pip install -r docsgpt/requirements.txt
 pip install 'pymongo>=4.6'
 
 export POSTGRES_URI="postgresql://docsgpt:docsgpt@localhost:5432/docsgpt"