refactor: create heartbeats func in HubStore

hub/internal/migrations/20260506182346_create_heartbeat_table.up.sql

@@ -1,10 +1,10 @@
 CREATE TABLE heartbeats (
     id INTEGER PRIMARY KEY AUTOINCREMENT,
     agent_id VARCHAR(32) UNIQUE NOT NULL,
-    cpu_usage FLOAT,
-    memory_usage FLOAT,
-    disk_usage FLOAT,
-    heartbeat_timestamp TIMESTAMP
+    cpu_usage FLOAT NOT NULL ,
+    memory_usage FLOAT NOT NULL ,
+    disk_usage FLOAT NOT NULL ,
+    heartbeat_timestamp TIMESTAMP NOT NULL,
     FOREIGN KEY (agent_id) REFERENCES agents(id) ON DELETE CASCADE
 );