Returns duration value or fallback on missing/parse failure.
Accepted units: - `s` seconds - `m` minutes - `h` hours - `d` days
Plain integer is treated as seconds.
| key |
config key. |
| fallback |
fallback duration. |
|
Result.ok(parsed duration or fallback), or Result.error(AppConfigError.INVALID_ARGUMENT) when key is empty. |