:sig
pid_t task_repeat(String key, f64 interval, std::function<void()> exec_func, u64 timeout = 60*10)

:params
key : string uniquely identifying the task across the whole runtime instance
interval : repeat interval in seconds
exec_func : function to execute repeatedly
timeout : maximum run time in seconds; `0` disables the timeout
return value : the process ID of the started (or still running) task, or `0` when the task could not be started

:see
>task
task
task_pid
task_kill

:content
Starts a repeating background worker process.

`exec_func` runs in a loop, and the worker sleeps for `interval` seconds between executions. `interval` must be greater than zero.

If a process with the same `key` is already running anywhere in the runtime instance, `task_repeat()` does not start a second worker and instead returns the PID of the existing one. Coordination is through the same shared task state used by `task()`.

`timeout` bounds the lifetime of the repeating worker. The default is ten minutes. Pass `0` only for workers that have another shutdown path.

:example
task_repeat("doc-demo-repeat", 60.0, []() { usleep(10000); });
pid_t pid = task_pid("doc-demo-repeat");
print(pid > 0 ? "repeating task scheduled" : "not scheduled", "\n");
if(pid > 0) task_kill(pid);