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_lancedb.py

@@ -6,13 +6,13 @@ import pytest
 def _make_lancedb_store(source_id="test-source"):
     """Helper to create a LanceDBVectorStore with mocked deps."""
     with patch(
-        "application.vectorstore.lancedb.settings"
+        "docsgpt.vectorstore.lancedb.settings"
     ) as mock_settings:
         mock_settings.LANCEDB_PATH = "/tmp/lancedb"
         mock_settings.LANCEDB_TABLE_NAME = "docs"
         mock_settings.EMBEDDINGS_NAME = "test_model"
 
-        from application.vectorstore.lancedb import LanceDBVectorStore
+        from docsgpt.vectorstore.lancedb import LanceDBVectorStore
 
         store = LanceDBVectorStore(
             path="/tmp/lancedb",
@@ -31,11 +31,11 @@ class TestLanceDBVectorStoreInit:
         assert store.table_name == "docs_src1"
 
     def test_table_name_without_source_id(self):
-        with patch("application.vectorstore.lancedb.settings") as mock_settings:
+        with patch("docsgpt.vectorstore.lancedb.settings") as mock_settings:
             mock_settings.LANCEDB_PATH = "/tmp"
             mock_settings.LANCEDB_TABLE_NAME = "docs"
 
-            from application.vectorstore.lancedb import LanceDBVectorStore
+            from docsgpt.vectorstore.lancedb import LanceDBVectorStore
 
             store = LanceDBVectorStore(
                 path="/tmp", table_name_prefix="docs", source_id=None