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/steam_watcher.rs

@@ -0,0 +1,53 @@
+use anyhow::Result;
+
+/// Steam App ID for the Rust Dedicated Server.
+pub const RUST_SERVER_APP_ID: u32 = 258550;
+
+/// Steam build ID polling service.
+///
+/// Periodically checks the Steam Web API for new Rust Dedicated Server
+/// builds. When a new build is detected (build ID changes), it publishes
+/// an event to NATS so that servers configured for auto-update-on-force-wipe
+/// can be notified and trigger their wipe schedules.
+pub struct SteamUpdateWatcher {
+    // TODO: Add fields:
+    // - nats: async_nats::Client
+    // - db: sqlx::PgPool
+    // - poll_interval: std::time::Duration
+    // - last_known_build_id: Option<String>
+}
+
+impl SteamUpdateWatcher {
+    /// Start the polling loop. Runs indefinitely, checking for updates
+    /// at the configured interval.
+    pub async fn start_polling(&self) -> Result<()> {
+        // TODO: Loop on poll_interval
+        // TODO: Call check_for_update each iteration
+        // TODO: If new build detected, call handle_update_detected
+        todo!()
+    }
+
+    /// Check Steam Web API for the current build ID of the Rust server app.
+    ///
+    /// Returns the build ID string if an update is detected (differs from
+    /// last known), or None if unchanged.
+    pub async fn check_for_update(&self) -> Result<Option<String>> {
+        // TODO: GET https://api.steampowered.com/ISteamApps/UpToDateCheck/v1/
+        //   ?appid={RUST_SERVER_APP_ID}&version=0
+        // TODO: Parse response for build ID
+        // TODO: Compare against last_known_build_id
+        // TODO: Return Some(new_id) if changed, None if same
+        todo!()
+    }
+
+    /// Handle a detected Steam update: persist the new build ID,
+    /// publish NATS event for downstream consumers.
+    pub async fn handle_update_detected(&self, _new_build_id: &str) -> Result<()> {
+        // TODO: Update last_known_build_id
+        // TODO: Persist to DB for crash recovery
+        // TODO: Publish to NATS subject corrosion.steam.update_detected
+        //   with payload { app_id, old_build_id, new_build_id, detected_at }
+        // TODO: Log the detection
+        todo!()
+    }
+}