build(android): release signing config — key.properties based, debug fallback

正式签名（CN=PhotoLive）：keystore/key.properties 不入库（gitignore 已覆盖）；
无配置的构建机回退 debug 签名。首个正式签名 APK 已产出并 apksigner 验证。

Co-Authored-By: Claude Code <noreply@anthropic.com>

apps/mobile/android/app/build.gradle.kts

@@ -1,3 +1,4 @@
+import java.util.Properties
 plugins {
     id("com.android.application")
     // The Flutter Gradle Plugin must be applied after the Android and Kotlin Gradle plugins.
@@ -25,11 +26,26 @@ android {
         versionName = flutter.versionName
     }
 
+    // 正式签名（key.properties + photo-live-release.jks，均不入库）：
+    // 缺失时回退 debug 签名（CI/他人克隆仓库仍可构建）。
+    val keystoreProps = Properties().apply {
+        val f = rootProject.file("app/key.properties")
+        if (f.exists()) f.inputStream().use { load(it) }
+    }
+
     buildTypes {
         release {
-            // TODO: Add your own signing config for the release build.
-            // Signing with the debug keys for now, so `flutter run --release` works.
-            signingConfig = signingConfigs.getByName("debug")
+            signingConfig = if (keystoreProps.getProperty("storeFile") != null) {
+                signingConfigs.create("release") {
+                    storeFile = rootProject.file("app/" + keystoreProps.getProperty("storeFile"))
+                    storePassword = keystoreProps.getProperty("storePassword")
+                    keyAlias = keystoreProps.getProperty("keyAlias")
+                    keyPassword = keystoreProps.getProperty("keyPassword")
+                }
+            } else {
+                // Signing with the debug keys（无正式签名配置的构建机回退）。
+                signingConfigs.getByName("debug")
+            }
             // R8 混淆会破坏 WorkManager 反射初始化（NoSuchMethodException
             // WorkDatabase_Impl），ProGuard keep 规则见 proguard-rules.pro。
             isMinifyEnabled = true