Replace credential auto-login with a Chrome-remote-debugging session capture flow

Live testing against the real site found MassPrice's login is behind
hCaptcha, which silently blocks fully automated username/password
login outright (the form never submits) — no selector fix could work
around this. Also fixed the broken _is_logged_out() heuristic: an
unauthenticated visit to /Rule/List redirects to the public marketing
homepage, not a /login URL, so run_rule/list_rules were silently
scraping the wrong page instead of erroring.

New flow: a Session page lets the user start a headless-but-CDP-exposed
Chromium instance, log in themselves via chrome://inspect (solving the
captcha as a real click), then capture that browser's cookies into the
same auth_state.json scheduled runs already use. Required a socat proxy
in the container image, since Chromium's debug port binds loopback-only
regardless of --remote-debugging-address (confirmed by live testing),
unreachable through Docker's port mapping without it. All Playwright
objects for this now live on one dedicated thread (automation.py's
_debug_worker), since the sync API isn't safe to call across the
arbitrary threads gunicorn's gthread workers hand out per request.

Dashboard now shows a banner when the latest run failed specifically
due to a missing/expired session, rather than just looking like an
empty rule list.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H6TgffxTvYhqxykAbVQg2v

templates/_shell.html

@@ -13,6 +13,7 @@
     <nav>
       <a href="{{ url_for('dashboard') }}" class="{{ 'active' if request.endpoint == 'dashboard' else '' }}">Dashboard</a>
       <a href="{{ url_for('activity') }}" class="{{ 'active' if request.endpoint == 'activity' else '' }}">Activity</a>
+      <a href="{{ url_for('session_page') }}" class="{{ 'active' if request.endpoint == 'session_page' else '' }}">Session</a>
       <a href="{{ url_for('logout') }}">Log Out</a>
     </nav>
     {% endif %}