feat: add API unit tests

Add Bun test suite covering:
- Auth: register, login, duplicate email, invalid password
- Events: create, location, validation, unauthorized
- Journals: get, generate, delete, list with pagination
- Settings: get, update, clear prompt, change password
- Export/Import: export data, events with location
- Search: search events, no matches, cross-date
- Days: list days, get day details, empty day

backend/package.json

@@ -9,7 +9,10 @@
     "db:generate": "prisma generate",
     "db:push": "prisma db push",
     "db:migrate": "prisma migrate dev",
-    "db:studio": "prisma studio"
+    "db:studio": "prisma studio",
+    "test": "bun test",
+    "test:watch": "bun test --watch",
+    "test:server": "bun run src/index.ts & sleep 2 && bun test && kill $!"
   },
   "dependencies": {
     "@prisma/client": "^5.22.0",