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.

docsgpt/cli.py

@@ -25,6 +25,49 @@ def _announce_home() -> None:
     print(f"docsgpt: data home {home_dir()} (env file {env_file()})", file=sys.stderr)
 
 
+def _gunicorn_options(host: str, port: int, workers: int) -> dict:
+    """The image's gunicorn flags (docsgpt/Dockerfile CMD), as settings."""
+    options = {
+        "bind": f"{host}:{port}",
+        "workers": workers,
+        "worker_class": "docsgpt.gunicorn_worker.BoundedDrainUvicornWorker",
+        "timeout": 180,
+        "graceful_timeout": 120,
+        "keepalive": 5,
+        "max_requests": 5000,
+        "max_requests_jitter": 500,
+    }
+    if os.path.isdir("/dev/shm"):
+        options["worker_tmp_dir"] = "/dev/shm"
+    return options
+
+
+def _gunicorn_application(options: dict):
+    """A gunicorn application configured in code rather than from sys.argv.
+
+    gunicorn records sys.argv at start and re-executes it on SIGUSR2 (the
+    zero-downtime upgrade), so sys.argv has to stay the ``docsgpt api ...``
+    invocation: the console script is what a re-exec must run again.
+    """
+    from gunicorn.app.base import Application
+
+    class DocsGPTApplication(Application):
+        def init(self, parser, opts, args):
+            return None
+
+        def load_config(self):
+            self.load_config_from_module_name_or_filename("python:docsgpt.gunicorn_conf")
+            for key, value in options.items():
+                self.cfg.set(key, value)
+
+        def load(self):
+            from docsgpt.asgi import asgi_app
+
+            return asgi_app
+
+    return DocsGPTApplication()
+
+
 def _api(args: argparse.Namespace) -> int:
     """Serve the ASGI app: gunicorn with the bounded-drain uvicorn worker, or uvicorn when reloading."""
     _announce_home()
@@ -34,25 +77,7 @@ def _api(args: argparse.Namespace) -> int:
         uvicorn.run("docsgpt.asgi:asgi_app", host=args.host, port=args.port, reload=args.reload)
         return 0
 
-    from gunicorn.app.wsgiapp import run as gunicorn_run
-
-    argv = [
-        "gunicorn",
-        "-w", str(args.workers),
-        "-k", "docsgpt.gunicorn_worker.BoundedDrainUvicornWorker",
-        "--bind", f"{args.host}:{args.port}",
-        "--timeout", "180",
-        "--graceful-timeout", "120",
-        "--keep-alive", "5",
-        "--max-requests", "5000",
-        "--max-requests-jitter", "500",
-        "--config", "python:docsgpt.gunicorn_conf",
-    ]
-    if os.path.isdir("/dev/shm"):
-        argv += ["--worker-tmp-dir", "/dev/shm"]
-    argv.append("docsgpt.asgi:asgi_app")
-    sys.argv = argv
-    gunicorn_run()
+    _gunicorn_application(_gunicorn_options(args.host, args.port, args.workers)).run()
     return 0