feat: publish the backend to PyPI as docsgpt

pip install docsgpt (extras: docling, milvus) installs the backend with a
docsgpt command: api, worker, migrate, prefetch-models, verify-offline,
reembed. Second step of the PyPI work after the package rename.

- hatchling build; the version comes from docsgpt/version.py. The wheel is
  the docsgpt package with the data it reads at runtime (prompts, model
  catalogs, seed config, alembic.ini and migrations) and without the
  Dockerfile, the exported requirements, the sample index and local runtime
  data. The application import alias stays checkout-only. uv sync installs
  the package editable now that [tool.uv] package = false is gone.
- docsgpt/cli.py: api (gunicorn + BoundedDrainUvicornWorker with the image's
  flags, --reload for uvicorn), worker (Celery worker with beat embedded,
  --no-beat/-Q/--concurrency/--pool, solo pool on macOS), migrate, and
  argument pass-through to the maintenance scripts. --help imports no app.
- docsgpt/core/paths.py: runtime data lives in a data home (DOCSGPT_HOME,
  else the checkout, else cwd); DOCSGPT_ENV_FILE overrides the env file.
  Settings, the dotenv load, LocalStorage and the internal upload route use
  it instead of "three directories above this file", which is site-packages
  for an installed package. A checkout and the Docker image behave as before.
- [project] dependencies are compatible ranges so the package installs next
  to other packages; uv.lock resolves to the same versions and the exported
  requirements files are unchanged.
- package-build.yml builds and checks the wheel on PRs and installs it into
  a clean venv; pypi-publish.yml publishes on a published release through
  trusted publishing (environment pypi), or to TestPyPI on a manual run.
- Docs: Deploying -> Install with pip. AGENTS.md notes the package.

docsgpt/core/paths.py

@@ -0,0 +1,47 @@
+"""Where DocsGPT keeps its files.
+
+Three locations matter. The package directory holds code and the data that
+ships with it (prompts, model catalogs, migrations). A checkout, when the
+package is imported from one, is the directory holding ``pyproject.toml``.
+The data home is where runtime data lives: ``.env``, ``inputs``, ``indexes``.
+
+The home is ``DOCSGPT_HOME`` when set, else the checkout, else the current
+directory. That keeps a source checkout and the Docker image (which runs from
+``/app`` with the package beside it) behaving as before, and gives a
+``pip install docsgpt`` user a home that is not ``site-packages``.
+"""
+
+from __future__ import annotations
+
+import os
+from pathlib import Path
+
+HOME_ENV = "DOCSGPT_HOME"
+ENV_FILE_ENV = "DOCSGPT_ENV_FILE"
+
+
+def package_dir() -> Path:
+    """The installed ``docsgpt`` package directory."""
+    return Path(__file__).resolve().parent.parent
+
+
+def checkout_root() -> Path | None:
+    """The source checkout the package is imported from, or None when installed."""
+    root = package_dir().parent
+    return root if (root / "pyproject.toml").is_file() else None
+
+
+def home_dir() -> Path:
+    """Directory for runtime data: ``DOCSGPT_HOME``, else the checkout, else cwd."""
+    configured = os.environ.get(HOME_ENV)
+    if configured:
+        return Path(configured).expanduser().resolve()
+    return checkout_root() or Path.cwd()
+
+
+def env_file() -> Path:
+    """The ``.env`` file settings load: ``DOCSGPT_ENV_FILE``, else ``<home>/.env``."""
+    configured = os.environ.get(ENV_FILE_ENV)
+    if configured:
+        return Path(configured).expanduser()
+    return home_dir() / ".env"