Android: sign release builds with our own upload key

A config plugin adds a release signing config that reads the key from
MUSICSHARE_UPLOAD_* in ~/.gradle/gradle.properties (kept out of the repo),
and falls back to the debug key where that isn't set up.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MzFywujuH9TiuXbr9vjoX8

apps/mobile/app.json

@@ -50,7 +50,8 @@
           "cameraPermission": "musicshare uses the camera to scan friends' QR codes.",
           "microphonePermission": false
         }
-      ]
+      ],
+      "./plugins/withReleaseSigning"
     ],
     "experiments": {
       "reactCompiler": true

apps/mobile/plugins/withReleaseSigning.js

@@ -0,0 +1,41 @@
+// Signs release builds with the musicshare upload key when it's configured in
+// ~/.gradle/gradle.properties (MUSICSHARE_UPLOAD_STORE_FILE, _STORE_PASSWORD,
+// _KEY_ALIAS, _KEY_PASSWORD); otherwise falls back to the debug key so builds
+// still work on other machines. The key itself never lives in the repo.
+const { withAppBuildGradle } = require('expo/config-plugins');
+
+const MARKER = '// musicshare release signing';
+
+const SIGNING_CONFIG = `
+        ${MARKER}
+        release {
+            if (project.hasProperty('MUSICSHARE_UPLOAD_STORE_FILE')) {
+                storeFile file(MUSICSHARE_UPLOAD_STORE_FILE)
+                storePassword MUSICSHARE_UPLOAD_STORE_PASSWORD
+                keyAlias MUSICSHARE_UPLOAD_KEY_ALIAS
+                keyPassword MUSICSHARE_UPLOAD_KEY_PASSWORD
+            }
+        }`;
+
+module.exports = function withReleaseSigning(config) {
+  return withAppBuildGradle(config, (cfg) => {
+    let gradle = cfg.modResults.contents;
+    if (gradle.includes(MARKER)) return cfg;
+
+    // Add a release signing config next to the debug one.
+    gradle = gradle.replace(/signingConfigs\s*\{/, (m) => `${m}${SIGNING_CONFIG}`);
+
+    // In the release build type, use it when the key is configured.
+    const release = gradle.indexOf('release {', gradle.indexOf('buildTypes {'));
+    if (release === -1) throw new Error('withReleaseSigning: no release build type in app/build.gradle');
+    const line = gradle.indexOf('signingConfig signingConfigs.debug', release);
+    if (line === -1) throw new Error('withReleaseSigning: release build type has no signingConfig line');
+    gradle =
+      gradle.slice(0, line) +
+      "signingConfig project.hasProperty('MUSICSHARE_UPLOAD_STORE_FILE') ? signingConfigs.release : signingConfigs.debug" +
+      gradle.slice(line + 'signingConfig signingConfigs.debug'.length);
+
+    cfg.modResults.contents = gradle;
+    return cfg;
+  });
+};