Add DeletePipeline API (#3506)

This is just a first step, the final goal is to have an API endpoint to
prune Repo Pipelines older than the given date.

@woodpecker-ci/maintainers Can I get some feedback if this is the right
direction?

---------

Co-authored-by: 6543 <m.huber@kithara.com>

server/api/helper.go

@@ -64,3 +64,14 @@ func refreshUserToken(c *gin.Context, user *model.User) {
 	}
 	forge.Refresh(c, _forge, _store, user)
 }
+
+// pipelineDeleteAllowed checks if the given pipeline can be deleted based on its status.
+// It returns a bool indicating if delete is allowed, and the pipeline's status.
+func pipelineDeleteAllowed(pl *model.Pipeline) bool {
+	switch pl.Status {
+	case model.StatusRunning, model.StatusPending, model.StatusBlocked:
+		return false
+	}
+
+	return true
+}