feat(auth): reset a forgotten password from an emailed link (milestone 497)

Step #5429 "Reset by email: request, link and new-password page" of
milestone 497 "Passwords: change your own, and reset it by email".

- Sign-in page: "Forgot your password?" while a mail server is set.
- /forgot takes a username or email and answers the same either way; only
  an active local account with a real address gets mail. Requests count
  per address on their own throttle, so they never lock out sign-in.
- password_resets (migration 0039): a one-time key, stored as its sha256,
  working for an hour; asking again replaces it.
- /reset/<key>: the new password, then signed straight in; every other
  browser is signed out (sessions_since). Bad keys count as invite keys do.
- Email "[Steward] Reset your password", in the account's own zone.
- Audited: user.password_reset_requested, user.password_reset_by_email.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/app.py

@@ -166,6 +166,8 @@ def create_app(
     from .auth import csrf as _csrf
     app.jinja_env.globals["csrf_token"] = _csrf.csrf_token
     app.jinja_env.globals["csrf_field"] = _csrf.csrf_field
+    # The sign-in page offers "Forgot your password?" only while mail can send (milestone 497).
+    app.jinja_env.globals["reset_by_email"] = lambda: bool(app.config.get("SMTP", {}).get("host"))
     app.before_request(_csrf.check_request)
     # The session cookie is Secure, and HSTS is sent, only on requests that
     # arrived over HTTPS — directly or as a trusted proxy reports it