feat: bootstrap backend MVP and architecture docs

app/services/__init__.py

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+"""Service layer."""

app/services/permission_service.py

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+from app.schemas.permissions import PermissionItem, PermissionSnapshotResponse
+
+
+class PermissionService:
+    @staticmethod
+    def build_snapshot(authentik_sub: str, permissions: list[tuple[str, str, str, str]]) -> PermissionSnapshotResponse:
+        return PermissionSnapshotResponse(
+            authentik_sub=authentik_sub,
+            permissions=[
+                PermissionItem(scope_type=s_type, scope_id=s_id, module=module, action=action)
+                for s_type, s_id, module, action in permissions
+            ],
+        )