Add mobile APK signing, build and release scripts

Add four new helper scripts for mobile APK workflows: setup-apk-signing.sh (decode keystores and export signing env vars), verify-apk-signature.sh (check and display APK certificate info), attach-apk-to-release.sh (rename and upload APK to a GitHub Release), and setup-mobile-github-secrets.sh (helper to generate/show required GitHub Secrets). Update product-release.yml to expose version/tag outputs and add a build-mobile-artifacts job that sets up Java/Flutter, installs deps, configures signing from repository secrets, builds APKs for worker/client apps, verifies signatures, uploads artifacts, and optionally attaches the APK to the GitHub Release. Secrets and envvar naming conventions are handled to support dev/staging/prod keystores; documentation references (docs/RELEASE/APK_SIGNING_SETUP.md) are noted in scripts.

.github/scripts/verify-apk-signature.sh

@@ -0,0 +1,59 @@
+#!/bin/bash
+
+# =============================================================================
+# Verify APK Signature
+# =============================================================================
+# This script verifies that an APK is properly signed and displays
+# certificate information
+#
+# Usage:
+#   ./verify-apk-signature.sh <apk_path>
+#
+# Arguments:
+#   apk_path - Path to the APK file to verify
+# =============================================================================
+
+set -e
+
+APK_PATH="$1"
+
+if [ -z "$APK_PATH" ]; then
+    echo "❌ Error: Missing APK path"
+    echo "Usage: $0 <apk_path>"
+    exit 1
+fi
+
+if [ ! -f "$APK_PATH" ]; then
+    echo "❌ APK not found at: $APK_PATH"
+    exit 1
+fi
+
+echo "🔍 Verifying APK signature..."
+
+# Check if APK is signed
+if jarsigner -verify -verbose "$APK_PATH" 2>&1 | grep -q "jar verified"; then
+    echo "✅ APK is properly signed!"
+    
+    # Extract certificate details
+    echo ""
+    echo "📜 Certificate Details:"
+    jarsigner -verify -verbose -certs "$APK_PATH" 2>&1 | grep -A 3 "X.509" || true
+    
+    # Get signer info
+    echo ""
+    echo "🔑 Signer Information:"
+    keytool -printcert -jarfile "$APK_PATH" | head -n 15
+    
+else
+    echo "⚠️  WARNING: APK signature verification failed or APK is unsigned!"
+    echo ""
+    echo "This may happen if:"
+    echo "  1. GitHub Secrets are not configured for this environment"
+    echo "  2. Keystore credentials are incorrect"
+    echo "  3. Build configuration didn't apply signing"
+    echo ""
+    echo "See: docs/RELEASE/APK_SIGNING_SETUP.md for setup instructions"
+    
+    # Don't fail the build, just warn
+    # exit 1
+fi