Replace per-request Chromium launch/teardown with a module-level
persistent browser. get_browser() lazy-initializes with is_connected()
auto-reconnect; shutdown_browser() is wired into FastAPI lifespan for
clean teardown. Pages are created per-request and closed in a finally
block to prevent leaks.
Also fixed pre-existing ruff errors in staged files (E402 noqa comments,
F541 f-string prefix removal, F841 unused variable rename) that were
blocking the pre-commit hook.
Closes#89
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
