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/schema/base.py

@@ -0,0 +1,34 @@
+"""Base schema for readers."""
+from dataclasses import dataclass
+
+from docsgpt.parser.schema.schema import BaseDocument
+from docsgpt.vectorstore.document_class import Document as VectorDocument
+
+
+@dataclass
+class Document(BaseDocument):
+    """Generic interface for a data document.
+
+    This document connects to data sources.
+
+    """
+
+    def __post_init__(self) -> None:
+        """Post init."""
+        if self.text is None:
+            raise ValueError("text field not set.")
+
+    @classmethod
+    def get_type(cls) -> str:
+        """Get Document type."""
+        return "Document"
+
+    def to_vector_format(self) -> VectorDocument:
+        """Convert struct to the page_content/metadata shape vector stores take."""
+        metadata = self.extra_info or {}
+        return VectorDocument(page_content=self.text, metadata=metadata)
+
+    @classmethod
+    def from_vector_format(cls, doc: VectorDocument) -> "Document":
+        """Convert struct from the vector-store document shape."""
+        return cls(text=doc.page_content, extra_info=doc.metadata)