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/error.py

@@ -0,0 +1,37 @@
+from flask import jsonify
+from werkzeug.http import HTTP_STATUS_CODES
+
+
+def response_error(code_status, message=None):
+    payload = {'error': HTTP_STATUS_CODES.get(code_status, "something went wrong")}
+    if message:
+        payload['message'] = message
+    response = jsonify(payload)
+    response.status_code = code_status
+    return response
+
+
+def bad_request(status_code=400, message=''):
+    return response_error(code_status=status_code, message=message)
+
+
+def sanitize_api_error(error) -> str:
+    """
+    Convert technical API errors to user-friendly messages.
+    Works with both Exception objects and error message strings.
+    """
+    error_str = str(error).lower()
+    if "503" in error_str or "unavailable" in error_str or "high demand" in error_str:
+        return "The AI service is temporarily unavailable due to high demand. Please try again in a moment."
+    if "429" in error_str or "rate limit" in error_str or "quota" in error_str:
+        return "Rate limit exceeded. Please wait a moment before trying again."
+    if "401" in error_str or "unauthorized" in error_str or "invalid api key" in error_str:
+        return "Authentication error. Please check your API configuration."
+    if "timeout" in error_str or "timed out" in error_str:
+        return "The request timed out. Please try again."
+    if "connection" in error_str or "network" in error_str:
+        return "Network error. Please check your connection and try again."
+    original = str(error)
+    if len(original) > 200 or "{" in original or "traceback" in error_str:
+        return "An error occurred while processing your request. Please try again later."
+    return original