Added PASSWORD_MIN_LENGTH and RESERVED_USERNAMES to src/constants.py as the
single source of truth. Previously PASSWORD_MIN_LENGTH was hardcoded as 8 in
four route handlers and all three JS validation paths; RESERVED_USERNAMES was
an inline frozenset duplicated in core/auth.py, routes/assistant_routes.py,
routes/research_routes.py, and src/task_scheduler.py.
Added GET /api/auth/policy (unauthenticated) so the frontend reads the real
values from the server instead of hardcoding them in JS.
Added missing empty-username guard to /setup and admin POST /users. Both
returned a misleading 500/409 on whitespace-only input. /signup already had the
check; this makes all three consistent.
Karl Jussila