task-65: transient payload 读取流式解压 + 解压后字节上限

- 新增 aiimage.transient-storage.max-decompressed-payload-bytes 配置
  （默认 100MB，即 2x 存储上限），覆盖 gzip 二进制与 gzip64 兼容两条路径
- decodeGzipStream 按 8KB 缓冲流式解压，累计输出超过上限立即中止并报错，
  防止压缩炸弹（zip bomb）在读取时无界膨胀内存
- 超限错误保留可识别消息（exceeds configured limit），由 resolvePayload 统一包装

backend-java/src/main/java/com/nanri/aiimage/config/TransientStorageProperties.java

@@ -37,6 +37,11 @@ public class TransientStorageProperties {
     private long warnPayloadBytes = 5L * 1024 * 1024;
     private long maxPayloadBytes = 50L * 1024 * 1024;
     private long maxStoredPayloadBytes = 50L * 1024 * 1024;
+    /**
+     * 读取端解压后字节上限：防止压缩炸弹（zip bomb）在流式解压时无界膨胀内存。
+     * 默认 2x 存储上限，覆盖 gzip 二进制路径与 gzip64 兼容路径。
+     */
+    private long maxDecompressedPayloadBytes = 100L * 1024 * 1024;
     private boolean fallbackToLocalOnOversize = true;
     private boolean deleteRetryEnabled = true;
     private String deleteRetryCron = "0 */5 * * * *";

backend-java/src/main/java/com/nanri/aiimage/modules/task/service/TransientPayloadStorageService.java

@@ -467,19 +467,15 @@ public class TransientPayloadStorageService {
             return "";
         }
         if (isGzip(storedContent)) {
-            try (GZIPInputStream gzip = new GZIPInputStream(new ByteArrayInputStream(storedContent))) {
-                return new String(gzip.readAllBytes(), StandardCharsets.UTF_8);
-            } catch (Exception ex) {
-                throw new IllegalStateException("failed to decode transient payload", ex);
-            }
+            return decodeGzipStream(storedContent);
         }
         String legacyText = new String(storedContent, StandardCharsets.UTF_8);
         if (legacyText.startsWith("gzip64:")) {
             try {
                 byte[] compressed = Base64.getDecoder().decode(legacyText.substring("gzip64:".length()));
-                try (GZIPInputStream gzip = new GZIPInputStream(new ByteArrayInputStream(compressed))) {
-                    return new String(gzip.readAllBytes(), StandardCharsets.UTF_8);
-                }
+                return decodeGzipStream(compressed);
+            } catch (IllegalStateException ex) {
+                throw ex;
             } catch (Exception ex) {
                 throw new IllegalStateException("failed to decode transient payload", ex);
             }
@@ -488,6 +484,37 @@ public class TransientPayloadStorageService {
         return legacyText;
     }
 
+    /**
+     * 流式解压 gzip 字节流，解压输出累计超过 maxDecompressedPayloadBytes 立即中止，
+     * 防止压缩炸弹在内存中无界膨胀。上限未配置（<=0）时不限流。
+     */
+    private String decodeGzipStream(byte[] compressed) {
+        long maxBytes = properties.getMaxDecompressedPayloadBytes();
+        try {
+            ByteArrayOutputStream out = new ByteArrayOutputStream();
+            try (GZIPInputStream gzip = new GZIPInputStream(new ByteArrayInputStream(compressed))) {
+                byte[] buffer = new byte[8192];
+                int read;
+                while ((read = gzip.read(buffer)) >= 0) {
+                    if (read == 0) {
+                        continue;
+                    }
+                    if (isPositiveLimit(maxBytes) && (long) out.size() + read > maxBytes) {
+                        throw new IllegalStateException(
+                                "transient payload decompressed size exceeds configured limit "
+                                        + maxBytes + " bytes");
+                    }
+                    out.write(buffer, 0, read);
+                }
+            }
+            return out.toString(StandardCharsets.UTF_8);
+        } catch (IllegalStateException ex) {
+            throw ex;
+        } catch (Exception ex) {
+            throw new IllegalStateException("failed to decode transient payload", ex);
+        }
+    }
+
     private boolean isGzip(byte[] content) {
         return content.length >= 2
                 && (content[0] & 0xFF) == 0x1F