Make session refresh retries idempotent

Preserve the successor session for legitimate refresh retries so transient failures no longer revoke the user's session family.

src/main/kotlin/com/osglab/account/config/AppConfig.kt

@@ -56,6 +56,10 @@ data class AppConfig(
                 hmacSecret = config.secret("app.session.secret", production).toByteArray(),
                 accessMinutes = config.positiveLong("app.session.accessMinutes"),
                 refreshDays = config.positiveLong("app.session.refreshDays"),
+                legacyRefreshReplaySeconds = config.positiveLong(
+                    "app.session.legacyRefreshReplaySeconds",
+                    30,
+                ),
                 gatewayGrantDays = config.positiveLong("app.session.gatewayGrantDays", 30),
             )
             val encryption = EncryptionConfig(
@@ -244,6 +248,9 @@ data class AppConfig(
             require(session.refreshDays in 1..365) {
                 "app.session.refreshDays must be between 1 and 365"
             }
+            require(session.legacyRefreshReplaySeconds in 5..120) {
+                "app.session.legacyRefreshReplaySeconds must be between 5 and 120"
+            }
             require(!production || providers.volcengine.credentialsAvailable) {
                 "Production Volcengine credentials are missing"
             }
@@ -380,6 +387,7 @@ data class SessionConfig(
     val hmacSecret: ByteArray,
     val accessMinutes: Long,
     val refreshDays: Long,
+    val legacyRefreshReplaySeconds: Long = 30,
     val gatewayGrantDays: Long = 30,
 )