envDuration accepted 0s and negative values, and
RETENTION_SWEEP_INTERVAL feeds time.NewTicker in both the retention
reaper and the archive sweeper. NewTicker panics on a non-positive
period, and both tickers are created in goroutines with no recover, so
a bad value aborted the process after startup had already logged
"Configuration loaded".
Add envPositiveDuration, mirroring envPort's wrapping of
envPositiveInt, and use it for RETENTION_SWEEP_INTERVAL. It wraps
ErrNonPositiveValue and names the variable, the same failure shape
PORT and RECEIVER_RATE_LIMIT already have.
SESSION_IDLE_TIMEOUT, the only other envDuration caller, stays on
envDuration: non-positive there means idle expiry is disabled, which
is documented behaviour and guarded at both use sites.
