Wraps the Task 2 CRUD layer in a FastAPI app (schemas.py + main.py) with
POST/GET/GET-by-id/PATCH routes for items, matching the exact paths and
status codes the Discord bot's HTTP client will depend on.
Also fixes a latent bug in db.get_engine: sqlite:///:memory: without a
StaticPool gives each new session a fresh, empty database, which broke
as soon as more than one session shared an engine (the API's per-request
session pattern). Tasks 1-2 never hit this because their tests used a
single session per engine.
