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/vectorstore/test_qdrant.py

@@ -51,12 +51,12 @@ def _settings(mock_settings, collection="test_collection"):
 
 @pytest.fixture
 def store():
-    from application.vectorstore.qdrant import QdrantStore
+    from docsgpt.vectorstore.qdrant import QdrantStore
 
     with patch(
-        "application.vectorstore.base.BaseVectorStore._get_embeddings",
+        "docsgpt.vectorstore.base.BaseVectorStore._get_embeddings",
         return_value=_FakeEmbeddings(),
-    ), patch("application.vectorstore.qdrant.settings") as mock_settings:
+    ), patch("docsgpt.vectorstore.qdrant.settings") as mock_settings:
         _settings(mock_settings)
         yield QdrantStore(source_id="src-A", embeddings_key="k")
 
@@ -135,12 +135,12 @@ class TestQdrantStore:
         assert len(populated.get_chunks()) == 2
 
     def test_delete_index_removes_only_this_source(self, populated):
-        from application.vectorstore.qdrant import QdrantStore
+        from docsgpt.vectorstore.qdrant import QdrantStore
 
         with patch(
-            "application.vectorstore.base.BaseVectorStore._get_embeddings",
+            "docsgpt.vectorstore.base.BaseVectorStore._get_embeddings",
             return_value=_FakeEmbeddings(),
-        ), patch("application.vectorstore.qdrant.settings") as mock_settings:
+        ), patch("docsgpt.vectorstore.qdrant.settings") as mock_settings:
             _settings(mock_settings)
             other = QdrantStore(source_id="src-B", embeddings_key="k")
         # Share the in-memory backend so both sources live in one collection.
@@ -159,9 +159,9 @@ class TestQdrantStore:
 @pytest.mark.unit
 class TestQdrantClientKwargs:
     def test_unset_settings_are_omitted(self):
-        from application.vectorstore.qdrant import QdrantStore
+        from docsgpt.vectorstore.qdrant import QdrantStore
 
-        with patch("application.vectorstore.qdrant.settings") as mock_settings:
+        with patch("docsgpt.vectorstore.qdrant.settings") as mock_settings:
             _settings(mock_settings)
             kwargs = QdrantStore._client_kwargs()
         # location/url/path are mutually exclusive in qdrant-client, so only
@@ -170,9 +170,9 @@ class TestQdrantClientKwargs:
         assert "url" not in kwargs and "path" not in kwargs and "host" not in kwargs
 
     def test_configured_settings_are_forwarded(self):
-        from application.vectorstore.qdrant import QdrantStore
+        from docsgpt.vectorstore.qdrant import QdrantStore
 
-        with patch("application.vectorstore.qdrant.settings") as mock_settings:
+        with patch("docsgpt.vectorstore.qdrant.settings") as mock_settings:
             _settings(mock_settings)
             mock_settings.QDRANT_LOCATION = None
             mock_settings.QDRANT_URL = "http://qdrant:6333"