feat: Add 15 backend service stub files with TODO implementations

Stub files for all services declared in services/mod.rs:
- Panel adapters: AMP, Pterodactyl, Companion (NATS-based)
- Wipe pipeline: WipeEngine, Scheduler, HealthChecker, BackupManager
- Infrastructure: NatsBridge (JetStream), SteamUpdateWatcher, MapManager
- Notifications: Discord webhooks, Pushbullet push notifications
- Platform: LicenseService, CloudflareService, encryption utilities

Each file has struct definitions, method signatures with proper types,
and descriptive TODO comments outlining the implementation plan.

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

backend/src/services/pushbullet.rs

@@ -0,0 +1,30 @@
+use anyhow::Result;
+
+/// Pushbullet notification service.
+///
+/// Sends push notifications to server administrators via the Pushbullet
+/// API. Used as a secondary notification channel alongside Discord for
+/// critical alerts that need to reach admins on their mobile devices.
+pub struct PushbulletNotifier {
+    // TODO: Add fields:
+    // - api_key: String
+    // - default_device_iden: Option<String> (target specific device, or all)
+}
+
+impl PushbulletNotifier {
+    /// Send a text notification (note type push).
+    pub async fn send_notification(&self, _title: &str, _body: &str) -> Result<()> {
+        // TODO: POST to https://api.pushbullet.com/v2/pushes
+        // TODO: Body: { type: "note", title, body }
+        // TODO: Auth: Access-Token header with api_key
+        todo!()
+    }
+
+    /// Send a link notification with a clickable URL.
+    pub async fn send_link(&self, _title: &str, _body: &str, _url: &str) -> Result<()> {
+        // TODO: POST to https://api.pushbullet.com/v2/pushes
+        // TODO: Body: { type: "link", title, body, url }
+        // TODO: Auth: Access-Token header with api_key
+        todo!()
+    }
+}