Task CRUD with validation and correct status codes

Full create, read, update and delete for Task, with input validation - required fields, string length limits, enum status values - and correct HTTP status codes. 201 on create, 404 on missing, 422 on validation failure. Not 200 and 500 everywhere.

Status codes are an API's error vocabulary. Collapsing everything into 200 and 500 means every client must parse prose to find out what happened, and retry logic becomes impossible to write correctly.