Add a Sinatra server that keeps boards in SQLite

Nullboard's storage layer bottoms out in getItem/setItem/delItem over
string keys, with boards, revisions and undo history all modelled on top
of them client-side. So the server needs to be nothing more than a
key/value store, and it doesn't parse a board anywhere.

One password, bcrypt-hashed, in the same database as the boards. The
session secret lives there too, so restarting the service doesn't sign
you out and the secret never has to exist in the unit file or in git.

Until a password is set every route returns 503 pointing at
bin/validboard-passwd. A first-run setup page would be friendlier, but it
would also hand the board to whoever found the URL first.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UU1vyTHj3uE9PJYSxRxwkU

config.ru

@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+
+require_relative 'app'
+
+run ValidBoard::App