feat: heartbeat (#108)

* feat: heartbeat

* feat: independent heartbeat model

db/queries/heartbeat_logs.sql

@@ -0,0 +1,25 @@
+-- name: CreateHeartbeatLog :one
+INSERT INTO bot_heartbeat_logs (bot_id, started_at)
+VALUES ($1, now())
+RETURNING id, bot_id, status, result_text, error_message, usage, started_at, completed_at;
+
+-- name: CompleteHeartbeatLog :one
+UPDATE bot_heartbeat_logs
+SET status = $2,
+    result_text = $3,
+    error_message = $4,
+    usage = $5,
+    completed_at = now()
+WHERE id = $1
+RETURNING id, bot_id, status, result_text, error_message, usage, started_at, completed_at;
+
+-- name: ListHeartbeatLogsByBot :many
+SELECT id, bot_id, status, result_text, error_message, usage, started_at, completed_at
+FROM bot_heartbeat_logs
+WHERE bot_id = $1
+  AND ($2::timestamptz IS NULL OR started_at < $2::timestamptz)
+ORDER BY started_at DESC
+LIMIT $3;
+
+-- name: DeleteHeartbeatLogsByBot :exec
+DELETE FROM bot_heartbeat_logs WHERE bot_id = $1;