Pre-commit hooks and CI linting for every commit

.githooks/pre-commit

.gitlab-ci.yml

backend/app/database.py

backend/app/hashing.py

backend/app/main.py

backend/app/test_main.py

frontend/.prettierignore

frontend/package-lock.json

frontend/package.json

frontend/postcss.config.js

frontend/rollup.config.js

frontend/src/lib/api/apiInstance.ts

frontend/src/lib/api/sessions.ts

frontend/src/lib/api/tests.ts

frontend/src/lib/api/users.ts

frontend/src/lib/components/header.svelte

frontend/src/lib/components/header/localSelector.svelte

frontend/src/lib/components/sessions/editParticipants.svelte

frontend/src/lib/components/sessions/message.svelte

frontend/src/lib/components/sessions/writebox.svelte