Add endpoint deletion

The endpoints screen could only create and edit servers, so a mistyped
or retired endpoint stayed in the list forever.

Tasks reference endpoints without ON DELETE CASCADE, so a referenced
endpoint is refused with 409 and a count of the tasks using it rather
than cascading away migration history. The foreign-key violation is
mapped to the same status to cover a task created between check and
delete.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

internal/httpapi/router.go

@@ -15,6 +15,7 @@ func (s *Server) Router() http.Handler {
 	api.HandleFunc("GET /api/endpoints", s.handleListEndpoints)
 	api.HandleFunc("POST /api/endpoints", s.handleCreateEndpoint)
 	api.HandleFunc("PUT /api/endpoints/{id}", s.handleUpdateEndpoint)
+	api.HandleFunc("DELETE /api/endpoints/{id}", s.handleDeleteEndpoint)
 	api.HandleFunc("GET /api/tasks", s.handleListTasks)
 	api.HandleFunc("POST /api/tasks", s.handleCreateTask)
 	api.HandleFunc("GET /api/tasks/{id}", s.handleGetTask)