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/core/optional_deps.py

@@ -0,0 +1,79 @@
+"""Optional dependency extras and the one place their install hints come from.
+
+Heavy or niche packages are not installed by default. Each extra maps to a
+pyproject extra and to an exported ``application/requirements-<extra>.txt``,
+so a missing module can always be explained with the exact command to run.
+"""
+
+from __future__ import annotations
+
+import importlib
+import importlib.util
+import sys
+from types import ModuleType
+from typing import Dict, Tuple
+
+#: Extra name -> top-level modules it provides. Keep in sync with
+#: ``[project.optional-dependencies]`` in pyproject.toml.
+EXTRAS: Dict[str, Tuple[str, ...]] = {
+    "docling": ("docling", "rapidocr"),
+    "milvus": ("pymilvus",),
+}
+
+_MODULE_TO_EXTRA: Dict[str, str] = {
+    module: extra for extra, modules in EXTRAS.items() for module in modules
+}
+
+
+def install_hint(extra: str) -> str:
+    """Install command for ``extra``, for error messages and logs."""
+    return (
+        f"pip install -r application/requirements-{extra}.txt "
+        f"(or: uv sync --extra {extra}; Docker: --build-arg EXTRAS={extra})"
+    )
+
+
+def extra_for(module: str) -> str | None:
+    """Extra that provides top-level module ``module``, if any."""
+    return _MODULE_TO_EXTRA.get(module.split(".")[0])
+
+
+def is_available(module: str) -> bool:
+    """Whether ``module`` can be imported, without importing it."""
+    root = module.split(".")[0]
+    if root in sys.modules:
+        return sys.modules[root] is not None
+    try:
+        return importlib.util.find_spec(root) is not None
+    except (ImportError, ValueError):
+        return False
+
+
+def missing_message(module: str, purpose: str | None = None) -> str:
+    """Human-readable explanation that ``module`` is absent and how to add it."""
+    extra = extra_for(module)
+    what = f"{module} is not installed"
+    if purpose:
+        what += f" ({purpose})"
+    if extra:
+        return f"{what}. It is part of the optional '{extra}' extra: {install_hint(extra)}"
+    return f"{what}. Install it with: pip install {module}"
+
+
+def require(module: str, purpose: str | None = None) -> ModuleType:
+    """Import ``module`` or raise ``ImportError`` naming the extra to install.
+
+    Args:
+        module: Importable module path, e.g. ``"pymilvus"``.
+        purpose: Short note on what needed it, included in the error.
+
+    Returns:
+        The imported module.
+
+    Raises:
+        ImportError: With the install hint when the module is absent.
+    """
+    try:
+        return importlib.import_module(module)
+    except ImportError as exc:
+        raise ImportError(missing_message(module, purpose)) from exc