Tasks creation and management

backend/app/crud/__init__.py

backend/app/crud/tasks.py

backend/app/main.py

backend/app/models/__init__.py

backend/app/models/tasks.py

backend/app/routes/tasks.py

backend/app/routes/tests.py

backend/app/schemas/__init__.py

backend/app/schemas/tasks.py

frontend/src/lang/fr.json

frontend/src/lib/api/tasks.ts

frontend/src/lib/components/studies/StudyForm.svelte

frontend/src/lib/types/tasks.ts

frontend/src/routes/Header.svelte

frontend/src/routes/admin/tasks/+page.svelte

frontend/src/routes/admin/tasks/+page.ts

frontend/src/routes/admin/tasks/TaskForm.svelte

frontend/src/routes/admin/tasks/[id]/+page.server.ts

frontend/src/routes/admin/tasks/[id]/+page.svelte

frontend/src/routes/admin/tasks/[id]/+page.ts