feat: add app-level authentication with login, session cookies, and admin panel

Replace Nginx basic auth with proper user accounts:
- Session cookies via itsdangerous (30-day expiry, httponly, secure)
- Password hashing with bcrypt via passlib
- Login page with AC-themed UI
- Admin page for user management (CRUD)
- AuthMiddleware exempts plugin WS and browser WS endpoints
- Issues/comments author auto-populated from session
- Sidebar shows logged-in username, admin link, and logout
- Seed users: erik (admin), alex, lundberg
- SECRET_KEY env var for cookie signing

Dockerfile

@@ -16,7 +16,9 @@ RUN python -m pip install --upgrade pip && \
         sqlalchemy \
         alembic \
         psycopg2-binary \
-        httpx
+        httpx \
+        passlib[bcrypt] \
+        itsdangerous
 
 ## Copy application source code and migration scripts into container
 COPY static/ /app/static/