Initial commit - Phase 3/4

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/backend/services/__init__.py

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+"""Service layer for Dangerous Pi.
+
+The service layer provides reusable business logic that can be consumed by
+multiple interfaces (REST API, BLE GATT, plugins, etc.).
+
+Services handle:
+- Business logic and validation
+- Session management
+- Error handling and formatting
+- Cross-cutting concerns
+
+This pattern prevents code duplication across interfaces.
+"""
+
+from .pm3_service import PM3Service, PM3ServiceResult, PM3ServiceError
+from .system_service import SystemService
+from .wifi_service import WiFiService
+from .update_service import UpdateService
+from .container import ServiceContainer, container
+
+__all__ = [
+    # PM3 Service
+    "PM3Service",
+    "PM3ServiceResult",
+    "PM3ServiceError",
+
+    # Other Services
+    "SystemService",
+    "WiFiService",
+    "UpdateService",
+
+    # Service Container
+    "ServiceContainer",
+    "container",
+]