regroup backend and frontend in app folder

app/backend/migrations/000001_init_schema.up.sql

@@ -0,0 +1,25 @@
+CREATE TABLE nodes (
+    id SERIAL PRIMARY KEY,
+    name VARCHAR(50) NOT NULL,
+    address VARCHAR(50) NOT NULL,
+    status SMALLINT NOT NULL,
+    last_seen TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE TABLE services (
+    id SERIAL PRIMARY KEY,
+    node_id INTEGER NOT NULL REFERENCES nodes(id) ON DELETE CASCADE,
+    name VARCHAR(50) NOT NULL,
+    status SMALLINT NOT NULL,
+    command TEXT
+);
+
+CREATE TABLE status_records (
+    id SERIAL PRIMARY KEY,
+    service_id INTEGER NOT NULL REFERENCES services(id) ON DELETE CASCADE,
+    status SMALLINT NOT NULL,
+    timestamp TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE INDEX idx_services_node_id ON services(node_id);
+CREATE INDEX idx_status_history ON status_records (service_id, timestamp DESC);