Add unit and integration tests

I won't be lying, Claude Code was extremely helpful to write those
tests, although it had a lot of issues with properly using in-memory
SQLite (db and integration tests) and with parsing responses
(integration test).

README.md

@@ -9,11 +9,14 @@ endpoints :
 * `POST /items`: creates an item
 * `GET /items`: returns the list of items
 
-## Usage
+## Running and testing
 
 This project uses Leiningen. Assuming you already have Leiningen installed,
 running the API should be as easy as cloning the repo and issuing `lein run`. By
-default, the server listens on port 3000.
+default, the server listens on port 3000. To explore the API using Swagger UI,
+you can head to <http://localhost:3000/doc>.
+
+To run the unit and integration tests: `lein test`
 
 ## Structure
 
@@ -48,3 +51,4 @@ The following links proved more than useful when working on this assignment:
 * <https://cljdoc.org/d/com.github.seancorfield/next.jdbc/1.3.1086/doc/getting-started>
 * <https://cljdoc.org/d/metosin/reitit/0.10.0/doc/ring/swagger-support>
 * <https://www.baeldung.com/clojure-ring>
+* <https://practical.li/clojure/testing/unit-testing/>