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/api/answer/services/compression/test_threshold_checker.py

@@ -7,18 +7,18 @@ import pytest
 class TestCompressionThresholdChecker:
 
     def _make_checker(self, pct=0.7):
-        from application.api.answer.services.compression.threshold_checker import (
+        from docsgpt.api.answer.services.compression.threshold_checker import (
             CompressionThresholdChecker,
         )
 
         return CompressionThresholdChecker(threshold_percentage=pct)
 
     @patch(
-        "application.api.answer.services.compression.threshold_checker.get_token_limit",
+        "docsgpt.api.answer.services.compression.threshold_checker.get_token_limit",
         return_value=8000,
     )
     @patch(
-        "application.api.answer.services.compression.threshold_checker.TokenCounter.count_message_tokens",
+        "docsgpt.api.answer.services.compression.threshold_checker.TokenCounter.count_message_tokens",
         return_value=6000,
     )
     def test_check_message_tokens_above_threshold(self, mock_count, mock_limit):
@@ -26,11 +26,11 @@ class TestCompressionThresholdChecker:
         assert checker.check_message_tokens([{"role": "user"}], "gpt-4") is True
 
     @patch(
-        "application.api.answer.services.compression.threshold_checker.get_token_limit",
+        "docsgpt.api.answer.services.compression.threshold_checker.get_token_limit",
         return_value=8000,
     )
     @patch(
-        "application.api.answer.services.compression.threshold_checker.TokenCounter.count_message_tokens",
+        "docsgpt.api.answer.services.compression.threshold_checker.TokenCounter.count_message_tokens",
         return_value=1000,
     )
     def test_check_message_tokens_below_threshold(self, mock_count, mock_limit):
@@ -38,7 +38,7 @@ class TestCompressionThresholdChecker:
         assert checker.check_message_tokens([{"role": "user"}], "gpt-4") is False
 
     @patch(
-        "application.api.answer.services.compression.threshold_checker.TokenCounter.count_message_tokens",
+        "docsgpt.api.answer.services.compression.threshold_checker.TokenCounter.count_message_tokens",
         side_effect=Exception("Token error"),
     )
     def test_check_message_tokens_exception_returns_false(self, mock_count):