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.

scripts/db/backfill_token_usage_model_id.py

@@ -1,7 +1,7 @@
 """Backfill ``token_usage.model_id`` for rows written before the column.
 
 New rows get ``model_id`` stamped at write time (see
-``application.llm.llm_creator`` / ``application.usage``). This script
+``docsgpt.llm.llm_creator`` / ``docsgpt.usage``). This script
 fills the historical NULLs by deriving the model from data we already
 trust, in priority order. A row is only ever filled by the
 highest-priority tier that matches it; tiers run in one transaction so
@@ -60,7 +60,7 @@ sys.path.insert(0, str(Path(__file__).resolve().parents[2]))
 
 from sqlalchemy import text  # noqa: E402
 
-from application.storage.db.engine import get_engine  # noqa: E402
+from docsgpt.storage.db.engine import get_engine  # noqa: E402
 
 
 # Tier 1: same request -> same model, primary (agent_stream) rows only.