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).

project.clj

@@ -12,7 +12,8 @@
                  [metosin/reitit-swagger-ui "0.10.0"]
                  [metosin/muuntaja "0.6.11"]
                  [com.github.seancorfield/next.jdbc "1.3.1086"]
-                 [org.xerial/sqlite-jdbc "3.51.1.0"]]
+                 [org.xerial/sqlite-jdbc "3.51.1.0"]
+                 [org.clojure/data.json "2.5.1"]]
   :main ^:skip-aot yohoho.app
   :target-path "target/%s"
   :profiles {:uberjar {:aot :all