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.

.github/workflows/docker-image-verify.yml

@@ -8,14 +8,15 @@ on:
   workflow_dispatch:
   pull_request:
     paths:
-      - 'application/Dockerfile'
-      - 'application/.dockerignore'
+      - 'docsgpt/Dockerfile'
+      - '.dockerignore'
+      - 'application/**'
       - 'application/requirements*.txt'
-      - 'application/scripts/prefetch_models.py'
-      - 'application/scripts/verify_offline.py'
-      - 'application/vectorstore/model_registry.py'
-      - 'application/parser/tokenization.py'
-      - 'application/vectorstore/embeddings_local.py'
+      - 'docsgpt/scripts/prefetch_models.py'
+      - 'docsgpt/scripts/verify_offline.py'
+      - 'docsgpt/vectorstore/model_registry.py'
+      - 'docsgpt/parser/tokenization.py'
+      - 'docsgpt/vectorstore/embeddings_local.py'
       - '.github/workflows/docker-image-verify.yml'
 
 permissions:
@@ -40,8 +41,8 @@ jobs:
       - name: Build the image
         uses: docker/build-push-action@10e90e3645eae34f1e60eeb005ba3a3d33f178e8 # v6.19.2
         with:
-          file: ./application/Dockerfile
-          context: ./application
+          file: ./docsgpt/Dockerfile
+          context: .
           platforms: linux/amd64
           load: true
           tags: docsgpt:verify${{ matrix.variant }}
@@ -63,4 +64,4 @@ jobs:
           IMAGE: docsgpt:verify${{ matrix.variant }}
         run: |
           docker run --rm --network none "$IMAGE" \
-            python -m application.scripts.verify_offline
+            python -m docsgpt.scripts.verify_offline