feat(package): ship the web UI in the wheel and serve it from the API

pip install docsgpt now brings the web UI with it: `docsgpt api` serves the
API and the UI on one port.

- scripts/build_frontend.sh builds the frontend into docsgpt/static
  (gitignored) the way the frontend image does: .env.development as the
  production baseline, and index.html loading /config.js ahead of the
  bundle. hatch admits the directory into the wheel and the sdist through
  `artifacts`; the package workflows run the script before `uv build` and
  fail if the wheel lacks the UI. The backend image keeps ignoring it.
- docsgpt/ui.py serves the build in front of Flask: files as they are,
  hashed assets immutable, Flask's own path prefixes (taken from its URL map,
  so new blueprints need no registration) passed through, every other GET
  rendered as index.html for the client-side router. /config.js is generated
  per request with VITE_API_HOST and VITE_BASE_URL set to the page's origin,
  VITE_* environment variables winning. SERVE_UI=false leaves the API alone.
- docsgpt api configures gunicorn in code (gunicorn.app.base.Application)
  instead of rewriting sys.argv, so the SIGUSR2 re-exec that gunicorn uses
  for zero-downtime upgrades runs the docsgpt console script again and
  works; verified with a live handover.
- Docs: the pip page says the UI is included, that DOCSGPT_HOME and
  DOCSGPT_ENV_FILE are process environment variables rather than .env
  entries, and the settings page describes SERVE_UI.

docs/content/Deploying/DocsGPT-Settings.mdx

@@ -69,6 +69,8 @@ Here are some of the most fundamental settings you'll likely want to configure:
 
 - **`STT_PROVIDER`**: Selects the speech-to-text provider used for microphone transcription in chat and for audio file ingestion through the parser pipeline.
 
+- **`SERVE_UI`**: Whether the API process serves the web UI shipped in the Python package (`pip install docsgpt`). Defaults to `true`; set `false` when a separate frontend, such as the Docker frontend image, serves the UI. Has no effect in the backend Docker image, which carries no UI build.
+
 ## Configuration Examples
 
 Let's look at some concrete examples of how to configure these settings in your `.env` file.

docs/content/Deploying/Pip-Install.mdx

@@ -7,10 +7,10 @@ import { Callout } from 'nextra/components'
 
 # Install with pip
 
-The DocsGPT backend is on PyPI as [`docsgpt`](https://pypi.org/project/docsgpt/): the API server, the Celery worker and the maintenance scripts in one package, behind a single `docsgpt` command. Use it when you want DocsGPT inside your own Python environment or process manager rather than the [Docker images](/Deploying/Docker-Deploying).
+DocsGPT is on PyPI as [`docsgpt`](https://pypi.org/project/docsgpt/): the API server, the web UI, the Celery worker and the maintenance scripts in one package, behind a single `docsgpt` command. Use it when you want DocsGPT inside your own Python environment or process manager rather than the [Docker images](/Deploying/Docker-Deploying).
 
 <Callout type="info">
-  The package is the backend only. The web UI ships in the Docker images and in the repository under `frontend/`; serving it from the package is planned.
+  `docsgpt api` serves the web UI on the same port as the API. Set `SERVE_UI=false` to run the API alone, for example behind the frontend Docker image or a UI you host yourself.
 </Callout>
 
 ## Requirements
@@ -49,7 +49,7 @@ pipx runpip docsgpt install --force-reinstall --no-deps --index-url https://down
 
 ## Configure
 
-DocsGPT keeps its runtime files in a **data home**: the `.env` file it reads settings from, uploaded files under `inputs/` and vector indexes under `indexes/`. The data home is the directory you run the commands from, or the directory `DOCSGPT_HOME` points to. `DOCSGPT_ENV_FILE` points at a `.env` kept somewhere else.
+DocsGPT keeps its runtime files in a **data home**: the `.env` file it reads settings from, uploaded files under `inputs/` and vector indexes under `indexes/`. The data home is the directory you run the commands from, or the directory `DOCSGPT_HOME` points to. `DOCSGPT_ENV_FILE` points at a `.env` kept somewhere else. Both variables must be set in the process environment, not in `.env`: they decide where `.env` is read from.
 
 Create a `.env` in the data home. The minimum for a hosted LLM:
 
@@ -72,7 +72,7 @@ Redis defaults to `localhost:6379` (`CELERY_BROKER_URL`, `CELERY_RESULT_BACKEND`
 
 ```bash
 docsgpt migrate    # create the database if it is missing and apply the migrations
-docsgpt api        # the API on http://127.0.0.1:7091
+docsgpt api        # the API and the web UI on http://127.0.0.1:7091
 docsgpt worker     # in a second terminal: the Celery worker, with the scheduler
 ```