Require a positive RETENTION_SWEEP_INTERVAL (closes #140)

A non-positive value reached time.NewTicker in the retention reaper and the archive sweeper, panicking both goroutines after startup had already reported success. envPositiveDuration now rejects it in loadFromEnv, matching how PORT and RECEIVER_RATE_LIMIT fail. SESSION_IDLE_TIMEOUT keeps treating non-positive as disabled, which is guarded at every use site.

README.md

@@ -93,6 +93,7 @@ TTY detection, and security headers are always applied.
 | `METRICS_USERNAME`      | Basic auth username for `/metrics`  | `""`     |
 | `METRICS_PASSWORD`      | Basic auth password for `/metrics`  | `""`     |
 | `SENTRY_DSN`            | Sentry error reporting DSN          | `""`     |
+| `RETENTION_SWEEP_INTERVAL` | Retention reaper period (Go duration, must be positive) | `1h` |
 | `SESSION_IDLE_TIMEOUT`  | Idle session timeout (Go duration)  | `24h`    |
 | `RECEIVER_RATE_LIMIT`   | Receiver requests/minute per IP per entrypoint | `120` |
 | `TRUSTED_PROXIES`       | CIDRs whose forwarded headers are trusted | `""` (none) |
@@ -173,8 +174,12 @@ its value and refuses to start, rather than silently running with a
 substituted default. `PORT=eighty`, `DEBUG=ture`, and
 `RETENTION_SWEEP_INTERVAL=1 hour` all abort startup. `PORT` must
 additionally be a number in the range 1–65535,
-`RECEIVER_RATE_LIMIT` must be at least 1, and every entry in
-`TRUSTED_PROXIES` must be a CIDR block or a bare IP address.
+`RECEIVER_RATE_LIMIT` must be at least 1,
+`RETENTION_SWEEP_INTERVAL` must be greater than zero (it is a ticker
+period, so `0s` or a negative value would crash the reaper after
+startup), and every entry in `TRUSTED_PROXIES` must be a CIDR block or
+a bare IP address. `SESSION_IDLE_TIMEOUT` is the exception: a
+non-positive value there means idle expiry is disabled, not invalid.
 
 Boolean variables (`DEBUG`, `MAINTENANCE_MODE`) accept exactly the
 spellings Go's `strconv.ParseBool` accepts — `1`, `t`, `T`, `TRUE`,