Kanban bucket limits (#652)

Fix integration tests

Generate swagger docs

Add test for moving a task between buckets

Add check for bucket limit when updating a task

Add fixture to ensure a bucket with a high limit will never exceed the limit

Refactor bucket limit check into seperate function

Add test for creating and fix

Fix unexported field

Add error in case a task was added to a bucket which has its limit already exceeded

Add migration to add new task field

Add limit field to buckets

Co-authored-by: kolaente <k@knt.li>
Reviewed-on: https://kolaente.dev/vikunja/api/pulls/652

pkg/models/error.go

@@ -1334,3 +1334,32 @@ func (err ErrCannotRemoveLastBucket) HTTPError() web.HTTPError {
 		Message:  "You cannot remove the last bucket on this list.",
 	}
 }
+
+// ErrBucketLimitExceeded represents an error where a task is being created or moved to a bucket which has its limit already exceeded.
+type ErrBucketLimitExceeded struct {
+	BucketID int64
+	Limit    int64
+	TaskID   int64 // may be 0
+}
+
+// IsErrBucketLimitExceeded checks if an error is ErrBucketLimitExceeded.
+func IsErrBucketLimitExceeded(err error) bool {
+	_, ok := err.(ErrBucketLimitExceeded)
+	return ok
+}
+
+func (err ErrBucketLimitExceeded) Error() string {
+	return fmt.Sprintf("Cannot add a task to this bucket because it would exceed the limit [BucketID: %d, Limit: %d, TaskID: %d]", err.BucketID, err.Limit, err.TaskID)
+}
+
+// ErrCodeBucketLimitExceeded holds the unique world-error code of this error
+const ErrCodeBucketLimitExceeded = 10004
+
+// HTTPError holds the http error description
+func (err ErrBucketLimitExceeded) HTTPError() web.HTTPError {
+	return web.HTTPError{
+		HTTPCode: http.StatusPreconditionFailed,
+		Code:     ErrCodeBucketLimitExceeded,
+		Message:  "You cannot add the task to this bucket as it already exceeded the limit of tasks it can hold.",
+	}
+}