Set up unit testing infrastructure

Extract slot generation from book.ts into pure utility for unit testing.
Add 8 API unit tests and 4 web component tests with coverage thresholds.

Closes #39

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/api/vitest.config.ts

@@ -2,6 +2,13 @@ import { defineConfig } from "vitest/config";
 
 export default defineConfig({
   test: {
-    passWithNoTests: true,
+    coverage: {
+      provider: "v8",
+      include: ["src/lib/**"],
+      thresholds: {
+        lines: 80,
+        functions: 80,
+      },
+    },
   },
 });