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.

pyproject.toml

@@ -170,11 +170,14 @@ dev = [
 path = "docsgpt/version.py"
 
 # The wheel is the docsgpt package with the data it reads at runtime: prompts,
-# model catalogs, the seed config, alembic.ini and the migrations. Build inputs
-# (Dockerfile, exported requirements), the sample index and local runtime data
-# stay out. The one-release `application` import alias is checkout-only.
+# model catalogs, the seed config, alembic.ini and the migrations, plus the web
+# UI build under docsgpt/static (gitignored; scripts/build_frontend.sh makes
+# it, and `artifacts` admits it). Build inputs (Dockerfile, exported
+# requirements), the sample index and local runtime data stay out. The
+# one-release `application` import alias is checkout-only.
 [tool.hatch.build.targets.wheel]
 packages = ["docsgpt"]
+artifacts = ["docsgpt/static/**"]
 exclude = [
   "docsgpt/Dockerfile",
   "docsgpt/requirements*.txt",
@@ -187,6 +190,7 @@ exclude = [
 
 [tool.hatch.build.targets.sdist]
 include = ["/docsgpt"]
+artifacts = ["docsgpt/static/**"]
 exclude = [
   "docsgpt/Dockerfile",
   "docsgpt/requirements*.txt",