feat: Firebase Cloud Messaging push notifications

Backend:
- New user_fcm_tokens table (migration 010) for storing device tokens
- FCMService sends push via Firebase HTTP legacy API (server key from .env)
- POST /api/fcm/register endpoint for Android to register its FCM token
- AdminController::sendNotification() triggers FCM push after DB insert
- FCM config added to config.php

Android:
- Firebase Cloud Messaging service (ServerManagerFirebaseService)
  receives push notifications and shows them via NotificationHelper
- FCM token registered with backend on new token and on login
- google-services.json template (must be replaced with real Firebase config)
- firebase-setup.sh script with configuration instructions
- FCM dependency + google-services plugin added to Gradle
- Bump to v1.8.0 (versionCode 12)

android/app/build.gradle.kts

@@ -2,6 +2,7 @@ plugins {
     id("com.android.application")
     id("org.jetbrains.kotlin.plugin.compose")
     id("org.jetbrains.kotlin.plugin.serialization")
+    id("com.google.gms.google-services")
 }
 
 android {
@@ -65,6 +66,8 @@ dependencies {
     implementation("androidx.core:core-splashscreen:1.0.1")
     implementation("androidx.work:work-runtime-ktx:2.10.0")
 
+    implementation("com.google.firebase:firebase-messaging-ktx:24.1.0")
+
     debugImplementation("androidx.compose.ui:ui-tooling")
     debugImplementation("androidx.compose.ui:ui-test-manifest")
 }