test(backend): implement complete test suite and minor endpoint fixes

test(backend): set up Vitest and Supertest and add health route tests

feat(auth): enhance cookie handling with dynamic SameSite and secure attributes

feat(cors): update accepted origins

feat(api): add health check endpoint to monitor service status

refactor: move payment endpoint from frontend to backend

fix(types): allow ProfileField type to be null to avoid build error

chore: approve pnpm builds

docs: update README to reflect project structure and remove Firebase references

chore: remove Firebase dependency

feat(payment): migrate payment service from Firebase to Prisma

feat(notes): migrate notes services from Firebase to Prisma

refactor: move auth endpoints to dedicated route

feat: migrate authentication from Firebase to Prisma

chore(backend): configure backend with prisma and express.js