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/parser/file/test_tableize.py

@@ -1,5 +1,5 @@
 """Tests for the dot-leader/whitespace table reconstruction (``ANYDOC_TABLEIZE``)."""
-from application.parser.file.tableize import tableize
+from docsgpt.parser.file.tableize import tableize
 
 DOT_LEADER = """Revenues
 Insurance premiums ............ 83,431 77,731
@@ -76,7 +76,7 @@ def test_non_table_text_passes_through_verbatim():
 
 def test_single_trailing_number_lines_are_not_a_table():
     """Headings, footnotes and version lists look like 'word number' rows; leave them alone."""
-    from application.parser.file.tableize import tableize
+    from docsgpt.parser.file.tableize import tableize
 
     for block in (
         "Chapter 1\nChapter 2\nChapter 3",
@@ -88,7 +88,7 @@ def test_single_trailing_number_lines_are_not_a_table():
 
 
 def test_leader_rows_with_one_value_still_convert():
-    from application.parser.file.tableize import tableize
+    from docsgpt.parser.file.tableize import tableize
 
     block = "Revenue ...... 1,234\nCosts ...... 567\nProfit ...... 667"
     assert "| --- |" in tableize(block)