Add a SQLite store backend (cl-sqlite)Verified

Third store backend implementing the full protocol (users, clients, auth-codes,
refresh, usher_credentials) over a single SQLite file, serialized by a lock
(SQLite is single-writer). Parameterized '?' queries; INTEGER unix-time and 0/1
booleans; atomic auth-code consume and transactional refresh rotate/reuse via
changes(). Selectable through config :db (:backend :sqlite :file "...").

Verified end-to-end against an in-process SQLite DB (test/sqlite-smoke.lisp,
18 checks). In-memory suite still 98/98.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>