fix: replace broken expo config plugin with post-prebuild Python patch

The expo config plugin approach failed because eas-cli (global install)
loads app.json plugins in its own module resolution context and cannot
find @expo/config-plugins from the project's local node_modules.

Replace with a Python3 inline script in build-apk.sh that patches
android/app/build.gradle after expo prebuild:
- Inserts a release signingConfig block (reads from gradle.properties)
- Switches release buildType from signingConfigs.debug to .release

Removes plugins/withAndroidReleaseSigning.js and its app.json entry.
No npm dependency required — Python3 is available on any Linux host.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

artifacts/postiz-mobile/app.json

@@ -48,7 +48,6 @@
           "sounds": []
         }
       ],
-      "./plugins/withAndroidReleaseSigning"
     ],
     "experiments": {
       "typedRoutes": true,