* fix: engine too many connections

Fix create_engine() connection leaks per request/call

* refactor: use lru cached engine

Use lru cached engine

* add pool_pre_ping and dispose()

* fix: address review feedback on engine cache

- Route rate_limit.py through the shared get_engine() cache instead of
  its own module-level singleton, so no second unmanaged pool survives
  clear_engine_cache() or goes stale when the connection URL changes.
- Guard the engine cache with a threading.Lock: sync dependencies run in
  FastAPI's threadpool, so concurrent first requests could race in
  get_engine() and orphan an undisposed engine, and clear_engine_cache()
  could hit a dict-mutated-during-iteration error.
- Remove the clear_engine_cache() side effect from tear_down_db(); test
  fixtures that relied on it now call it explicitly in their teardown.
- Make engine-cache tests isolation-safe via an engine_cache fixture so
  cleanup runs even when an assertion fails mid-test.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Rafiuzzaman Khan <rafiuzzamank@gmail.com>
Co-authored-by: chriscarrollsmith <chriscarrollsmith@gmail.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

…ng conflicts

Keep main's shared engine cache and clear_engine_cache lifespan cleanup
alongside stripe billing imports, and retain sync_default_role_permissions
in set_up_db with try/finally dispose.

Co-authored-by: Christopher Carroll Smith <chriscarrollsmith@users.noreply.github.com>