feat(server): FastAPI app with API-key auth and health check

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/conftest.py

@@ -0,0 +1,16 @@
+import os
+import pytest
+
+os.environ.setdefault("MUSICFETCH_API_KEY", "test-key")
+
+
+@pytest.fixture
+def client():
+    from fastapi.testclient import TestClient
+    from server.app import app
+    return TestClient(app)
+
+
+@pytest.fixture
+def auth():
+    return {"X-API-Key": "test-key"}