feat: add retry with exponential backoff to all repositories

- New: RetryHelper.kt — retryOnTransientError(maxRetries=2, baseDelay=1s)
- Wraps in RefreshBalanceUseCase — single dispatch point for all 5 services
- Retries on Network and 5xx ServerError
- 401/403 (InvalidCredential), 429 (RateLimited), ParseError — no retry

app/src/main/java/com/rainy/token/data/repository/RetryHelper.kt

@@ -0,0 +1,39 @@
+package com.rainy.token.data.repository
+
+import kotlinx.coroutines.delay
+import kotlin.math.pow
+
+/**
+ * 通用重试包装。
+ *
+ * 对 [RepositoryError.Network] 和 [RepositoryError.ServerError] 执行指数退避重试。
+ * 401/403（InvalidCredential）、429（RateLimited）、ParseError 等不重试。
+ *
+ * @param maxRetries 最大重试次数（不含首次），默认 2
+ * @param baseDelayMs 退避基数，默认 1000ms（第二次 = 2000ms）
+ * @param block 实际请求逻辑
+ */
+suspend fun <T> retryOnTransientError(
+    maxRetries: Int = 2,
+    baseDelayMs: Long = 1000L,
+    block: suspend () -> Result<T>
+): Result<T> {
+    var lastResult = block()
+    if (lastResult.isSuccess) return lastResult
+
+    repeat(maxRetries) { attempt ->
+        val error = lastResult.exceptionOrNull()
+        val shouldRetry = when (error) {
+            is RepositoryError.Network -> true
+            is RepositoryError.ServerError -> error.code in 500..599
+            else -> false
+        }
+        if (!shouldRetry) return lastResult
+
+        val backoffMs = baseDelayMs * (2.0.pow(attempt).toLong())
+        delay(backoffMs)
+        lastResult = block()
+        if (lastResult.isSuccess) return lastResult
+    }
+    return lastResult
+}

app/src/main/java/com/rainy/token/domain/usecase/RefreshBalanceUseCase.kt

@@ -5,6 +5,7 @@ import com.rainy.token.data.repository.CommandCodeGoRepository
 import com.rainy.token.data.repository.DeepSeekRepository
 import com.rainy.token.data.repository.OllamaRepository
 import com.rainy.token.data.repository.OpenCodeGoRepository
+import com.rainy.token.data.repository.retryOnTransientError
 import com.rainy.token.domain.model.ServiceBalance
 import com.rainy.token.domain.service.ServiceType
 import javax.inject.Inject
@@ -19,6 +20,8 @@ import javax.inject.Provider
  *    注入同一 UseCase 时的"could not be resolved"误报（KSP 已知 issue）。
  *    Provider 让 Hilt 推迟创建 Repository 实例到第一次 .get() 时，KSP 不需要在
  *    编译期解析所有构造器签名。
+ *  - **重试策略**：对 Network 和 5xx ServerError 自动指数退避重试（最多 2 次），
+ *    401/403/429/ParseError 不重试。
  */
 class RefreshBalanceUseCase @Inject constructor(
     private val deepSeekRepositoryProvider: Provider<DeepSeekRepository>,
@@ -28,10 +31,10 @@ class RefreshBalanceUseCase @Inject constructor(
     private val ollamaRepositoryProvider: Provider<OllamaRepository>
 ) {
     suspend operator fun invoke(service: ServiceType): Result<ServiceBalance> = when (service) {
-        ServiceType.DEEPSEEK -> deepSeekRepositoryProvider.get().fetchBalance()
+        ServiceType.DEEPSEEK -> retryOnTransientError { deepSeekRepositoryProvider.get().fetchBalance() }
-        ServiceType.OPENCODE_GO -> openCodeGoRepositoryProvider.get().fetchBalance()
+        ServiceType.OPENCODE_GO -> retryOnTransientError { openCodeGoRepositoryProvider.get().fetchBalance() }
-        ServiceType.COMMANDCODE_GO -> commandCodeGoRepositoryProvider.get().fetchBalance()
+        ServiceType.COMMANDCODE_GO -> retryOnTransientError { commandCodeGoRepositoryProvider.get().fetchBalance() }
-        ServiceType.CODEX -> codexRepositoryProvider.get().fetchBalance()
+        ServiceType.CODEX -> retryOnTransientError { codexRepositoryProvider.get().fetchBalance() }
-        ServiceType.OLLAMA -> ollamaRepositoryProvider.get().fetchBalance()
+        ServiceType.OLLAMA -> retryOnTransientError { ollamaRepositoryProvider.get().fetchBalance() }
     }
 }