Security: document WEED_TLS_CERT_REFRESH_INTERVAL for tuning refresh window

Added a user-facing env var that overrides the 5h default refresh cadence
for both gRPC mTLS and HTTPS cert reloading, for operators running short-
lived certs (Vault, cert-manager with 24h leaves, etc.).

Security-Configuration.md

@@ -260,7 +260,17 @@ New TLS handshakes pick up the rotated cert. In-flight connections keep the cert
 
 **CA rotation caveat.** Only leaf cert/key pairs hot-reload. CA bundles (`[grpc.ca]`, `[https.*.ca]`, `[https.client.ca]`) are loaded once at startup. In k8s, cert-manager typically rotates leaf certs far more often than CAs — if your CA rolls, you still need a rolling restart.
 
-**How it works.** A `GetCertificate` callback is wired into every server-side `tls.Config`. Under the hood it's backed by gRPC's `pemfile.NewProvider`, which stats the cert/key files on its refresh tick and re-parses them only when `mtime` or contents change. The refresh interval is `security.CredRefreshingInterval` (currently 5 hours). To shorten it, fork and change that constant — a flag for this is a reasonable future addition.
+**How it works.** A `GetCertificate` callback is wired into every server-side `tls.Config`. Under the hood it's backed by gRPC's `pemfile.NewProvider`, which stats the cert/key files on its refresh tick and re-parses them only when `mtime` or contents change. The default refresh interval is 5 hours.
+
+**Tuning the refresh window.** Set the environment variable `WEED_TLS_CERT_REFRESH_INTERVAL` (accepts any `time.ParseDuration` value, e.g. `30m`, `5m`, `500ms`) on every `weed` process that should pick up rotations faster. Useful with short-lived certs (Vault, cert-manager with `duration: 24h`, etc.). Example:
+
+```yaml
+env:
+  - name: WEED_TLS_CERT_REFRESH_INTERVAL
+    value: "1m"
+```
+
+The same env var tunes both gRPC mTLS and HTTPS reload.
 
 **Requirements for hot reload to work:**