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/license.rs

@@ -0,0 +1,67 @@
+use anyhow::Result;
+use uuid::Uuid;
+
+use crate::models::license::License;
+
+/// License validation and lifecycle management.
+///
+/// Handles license key validation, activation (binding a key to a server),
+/// periodic check-ins from the Rust plugin, and license lookups.
+/// All license state is stored in PostgreSQL.
+pub struct LicenseService {
+    // TODO: Add fields:
+    // - db: sqlx::PgPool
+}
+
+impl LicenseService {
+    /// Validate a license key and return its current status.
+    ///
+    /// Checks: key exists, not expired, not revoked, modules enabled.
+    pub async fn validate_license(&self, _license_key: &str) -> Result<Option<License>> {
+        // TODO: Query license by key from DB
+        // TODO: Check status is 'active'
+        // TODO: Check expires_at is in the future (if set)
+        // TODO: Return Some(license) if valid, None if not found
+        todo!()
+    }
+
+    /// Activate a license key: bind it to a server name and subdomain.
+    ///
+    /// Called during initial setup when a user first registers.
+    pub async fn activate_license(
+        &self,
+        _license_key: &str,
+        _server_name: &str,
+        _subdomain: &str,
+    ) -> Result<License> {
+        // TODO: Validate license key exists and is in 'pending' status
+        // TODO: Check subdomain availability
+        // TODO: Update license record: status='active', server_name, subdomain
+        // TODO: Create default roles for this license
+        // TODO: Return updated license
+        todo!()
+    }
+
+    /// Process a check-in from the Rust server plugin.
+    ///
+    /// Updates last-seen timestamp and returns current license state
+    /// including enabled modules and NATS connection token.
+    pub async fn check_in(
+        &self,
+        _license_key: &str,
+        _plugin_version: &str,
+    ) -> Result<crate::models::license::LicenseCheckInResponse> {
+        // TODO: Validate license
+        // TODO: Update plugin_last_seen timestamp on server_connections
+        // TODO: Generate or refresh NATS auth token for this license
+        // TODO: Return LicenseCheckInResponse with modules and token
+        todo!()
+    }
+
+    /// Look up a license by its UUID.
+    pub async fn get_license_by_key(&self, _license_id: Uuid) -> Result<Option<License>> {
+        // TODO: Query license by ID from DB
+        // TODO: Return if found
+        todo!()
+    }
+}