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/file/constants.py

@@ -0,0 +1,102 @@
+"""Shared file-extension constants for parsing and ingestion flows."""
+
+import os
+
+from docsgpt.parser.file.anydoc_parser import ANYDOC_GAINED_SUFFIXES
+from docsgpt.stt.constants import SUPPORTED_AUDIO_EXTENSIONS
+
+
+SUPPORTED_SOURCE_DOCUMENT_EXTENSIONS = (
+    ".rst",
+    ".md",
+    ".pdf",
+    ".txt",
+    ".docx",
+    ".csv",
+    ".epub",
+    ".html",
+    ".mdx",
+    ".json",
+    ".xlsx",
+    ".pptx",
+    # Read by the HTML parsers on every engine.
+    ".xhtml",
+    # Read by the anydoc engine (legacy/macro Office, OpenDocument, RTF).
+    # Parseable regardless of DOC_PARSER_ENGINE: anydoc is a core dependency,
+    # and both parser maps route these suffixes to it.
+    *ANYDOC_GAINED_SUFFIXES,
+)
+
+SUPPORTED_SOURCE_IMAGE_EXTENSIONS = (".png", ".jpg", ".jpeg")
+
+SUPPORTED_SOURCE_EXTENSIONS = (
+    *SUPPORTED_SOURCE_DOCUMENT_EXTENSIONS,
+    *SUPPORTED_SOURCE_IMAGE_EXTENSIONS,
+    *SUPPORTED_AUDIO_EXTENSIONS,
+)
+
+# Suffixes the attachment path has a dedicated parser for — exactly the keys
+# of ``get_default_file_extractor()``. Kept as a literal (importing ``bulk``
+# here would drag docling into the API process), with
+# ``tests/parser/file/test_constants.py`` asserting the two agree.
+#
+# This is *not* the whole attachment allow-list: a suffix with no parser is
+# read by ``SimpleDirectoryReader``'s plain-text fallthrough, which is right
+# for a .py or a .log and catastrophic for a video — the reader "extracts"
+# megabytes of binary garbage, truncates it, and stores it with
+# ``extraction.status == "ok"``. So unparsed suffixes are admitted on
+# content instead (``upload_limits.enforce_parseable_attachment``): text
+# passes, binary is refused. Zip is deliberately absent — source ingestion
+# extracts archives, the attachment path does not, and a zip fails the
+# content check like any other binary.
+#
+# Mirrored in ``frontend/src/constants/fileUpload.ts``; update both together.
+ATTACHMENT_PARSER_EXTENSIONS = frozenset(
+    {
+        *SUPPORTED_SOURCE_EXTENSIONS,
+        ".xhtml",
+        ".adoc",
+        ".asciidoc",
+        ".tiff",
+        ".tif",
+        ".bmp",
+        ".webp",
+        ".vtt",
+        ".xml",
+    }
+    # .txt has no parser of its own — it *is* the plain-text fallthrough. It
+    # must be sniffed like any other unparsed suffix, or renaming a video to
+    # notes.txt walks straight back into the bug this gate exists for.
+    - {".txt"}
+)
+
+
+def attachment_extension(filename: str | None) -> str:
+    """Return the lower-cased extension of ``filename`` including the dot, or ``""``.
+
+    Args:
+        filename: A bare filename or path; ``None`` and empty strings yield ``""``.
+
+    Returns:
+        The last suffix in lower case (``".pdf"``), or ``""`` when there is none.
+    """
+    if not filename:
+        return ""
+    return os.path.splitext(os.path.basename(str(filename)))[1].lower()
+
+
+def has_attachment_parser(filename: str | None) -> bool:
+    """Return whether an attachment's suffix has a dedicated parser.
+
+    A False result does not mean the file is refused: it means nothing but the
+    plain-text fallthrough will read it, so it has to earn its place on
+    content. The decision is by extension only, never by the mime type a
+    browser reports (mobile pickers ignore ``accept`` and lie).
+
+    Args:
+        filename: The upload's filename.
+
+    Returns:
+        True when the suffix is in ``ATTACHMENT_PARSER_EXTENSIONS``.
+    """
+    return attachment_extension(filename) in ATTACHMENT_PARSER_EXTENSIONS