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.

docsgpt/parser/chunking_creator.py

@@ -0,0 +1,57 @@
+"""String-keyed registry for chunking strategies.
+
+Mirrors ``RetrieverCreator``: features register new strategies (``recursive``,
+``markdown``, ``parent_child``, ...) without touching the dispatch site. The
+classic strategy is registered under ``classic_chunk`` by ``chunking.py``.
+"""
+
+from __future__ import annotations
+
+from typing import Type
+
+
+class ChunkerCreator:
+    chunkers: dict[str, Type] = {}
+    _strategies_loaded: bool = False
+
+    @classmethod
+    def _ensure_builtin(cls) -> None:
+        """Register built-in chunkers if they are not registered yet.
+
+        Self-bootstraps so ``create_chunker`` works regardless of import order:
+        ``docsgpt.parser.chunking`` registers ``classic_chunk`` and
+        ``docsgpt.parser.chunking_strategies`` registers ``recursive`` /
+        ``markdown`` / ``parent_child``.
+        """
+        if not cls.chunkers:
+            import docsgpt.parser.chunking  # noqa: F401  (registers classic_chunk)
+        if not cls._strategies_loaded:
+            cls._strategies_loaded = True
+            import docsgpt.parser.chunking_strategies  # noqa: F401
+
+    @classmethod
+    def create_chunker(cls, strategy: str, *args, **kwargs):
+        """Instantiate the chunker registered under ``strategy``.
+
+        Args:
+            strategy: Registry key (e.g. ``classic_chunk``).
+            *args: Positional args forwarded to the chunker constructor.
+            **kwargs: Keyword args forwarded to the chunker constructor.
+
+        Returns:
+            A chunker instance exposing ``chunk(documents) -> List[Document]``.
+
+        Raises:
+            ValueError: If no chunker is registered for ``strategy``.
+        """
+        cls._ensure_builtin()
+        key = (strategy or "classic_chunk").lower()
+        chunker_class = cls.chunkers.get(key)
+        if not chunker_class:
+            raise ValueError(f"No chunker class found for strategy {strategy}")
+        return chunker_class(*args, **kwargs)
+
+    @classmethod
+    def register(cls, key: str, chunker_class: Type) -> None:
+        """Register ``chunker_class`` under ``key`` (idempotent)."""
+        cls.chunkers[key] = chunker_class