refactor: align backend layout and supabase infra

Consolidate backend modules/tests under the backend package while syncing Supabase compose/env config and related plans.

backend/tests/unit/services/base/test_service_registry.py

@@ -0,0 +1,49 @@
+from __future__ import annotations
+
+from services.base.service_interface import (
+    BaseServiceProvider,
+    ServiceRegistry,
+    register_service,
+    register_service_instance,
+)
+
+
+class _DummyService(BaseServiceProvider):
+    def __init__(self, name: str = "dummy") -> None:
+        super().__init__(name)
+
+    async def initialize(self, **_: object) -> bool:
+        self._set_initialized(True)
+        return True
+
+    async def close(self) -> bool:
+        self._set_initialized(False)
+        return True
+
+    async def health_check(self) -> dict[str, object]:
+        return {"status": "healthy", "details": {}}
+
+
+def test_register_service_and_create_service() -> None:
+    @register_service("dummy-service")
+    class _RegisteredService(_DummyService):
+        pass
+
+    created = ServiceRegistry.create_service("dummy-service")
+
+    assert created is not None
+    assert created.get_service_info()["name"] == "dummy"
+
+
+def test_register_service_instance_returns_same_instance() -> None:
+    instance = _DummyService("singleton")
+
+    returned = register_service_instance("dummy-singleton", instance)
+    created = ServiceRegistry.create_service("dummy-singleton")
+
+    assert returned is instance
+    assert created is instance
+
+
+def test_create_service_returns_none_for_missing() -> None:
+    assert ServiceRegistry.create_service("missing-service") is None