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/companion_adapter.rs

@@ -0,0 +1,92 @@
+use anyhow::Result;
+use async_trait::async_trait;
+use uuid::Uuid;
+
+use super::panel_adapter::{DiscoveredServer, FileEntry, PanelAdapter, ServerStatus};
+
+/// Companion Agent adapter.
+///
+/// Unlike AMP and Pterodactyl which use REST APIs, the Companion Agent
+/// runs alongside the game server and communicates via NATS JetStream.
+/// Commands are published to the agent's subject namespace and responses
+/// are received on reply subjects. This enables direct server management
+/// without a panel intermediary.
+pub struct CompanionAdapter {
+    pub nats: async_nats::Client,
+    pub license_id: Uuid,
+}
+
+impl CompanionAdapter {
+    pub fn new(nats: async_nats::Client, license_id: Uuid) -> Self {
+        Self { nats, license_id }
+    }
+}
+
+#[async_trait]
+impl PanelAdapter for CompanionAdapter {
+    async fn test_connection(&self) -> Result<bool> {
+        // TODO: Publish heartbeat request to corrosion.{license_id}.agent.ping,
+        // await reply within timeout
+        todo!()
+    }
+
+    async fn discover_servers(&self) -> Result<Vec<DiscoveredServer>> {
+        // TODO: Request server info from agent via NATS request/reply.
+        // Companion manages exactly one server, so this returns a single-element vec.
+        todo!()
+    }
+
+    async fn get_server_status(&self, _server_id: &str) -> Result<ServerStatus> {
+        // TODO: NATS request to corrosion.{license_id}.agent.status
+        todo!()
+    }
+
+    async fn start_server(&self, _server_id: &str) -> Result<()> {
+        // TODO: NATS request to corrosion.{license_id}.agent.start
+        todo!()
+    }
+
+    async fn stop_server(&self, _server_id: &str) -> Result<()> {
+        // TODO: NATS request to corrosion.{license_id}.agent.stop
+        todo!()
+    }
+
+    async fn restart_server(&self, _server_id: &str) -> Result<()> {
+        // TODO: NATS request to corrosion.{license_id}.agent.restart
+        todo!()
+    }
+
+    async fn send_command(&self, _server_id: &str, _command: &str) -> Result<String> {
+        // TODO: NATS request to corrosion.{license_id}.agent.command
+        // with command payload, await RCON response
+        todo!()
+    }
+
+    async fn get_file(&self, _server_id: &str, _path: &str) -> Result<Vec<u8>> {
+        // TODO: NATS request to corrosion.{license_id}.agent.file.get
+        // Agent reads local file and returns contents
+        todo!()
+    }
+
+    async fn put_file(&self, _server_id: &str, _path: &str, _data: &[u8]) -> Result<()> {
+        // TODO: NATS publish to corrosion.{license_id}.agent.file.put
+        // May need chunking for large files
+        todo!()
+    }
+
+    async fn delete_file(&self, _server_id: &str, _path: &str) -> Result<()> {
+        // TODO: NATS request to corrosion.{license_id}.agent.file.delete
+        todo!()
+    }
+
+    async fn list_files(&self, _server_id: &str, _path: &str) -> Result<Vec<FileEntry>> {
+        // TODO: NATS request to corrosion.{license_id}.agent.file.list
+        todo!()
+    }
+
+    async fn trigger_steam_update(&self, _server_id: &str) -> Result<()> {
+        // TODO: NATS request to corrosion.{license_id}.agent.update
+        // Agent runs SteamCMD locally
+        todo!()
+    }
+}