code refactor

Reviewed-on: #26

crates/adapters/sqlite/migrations/0100_event_queue.sql

@@ -0,0 +1,13 @@
+CREATE TABLE IF NOT EXISTS event_queue (
+    id              INTEGER PRIMARY KEY AUTOINCREMENT,
+    event_type      TEXT    NOT NULL,
+    payload         TEXT    NOT NULL,
+    status          TEXT    NOT NULL DEFAULT 'pending',
+    attempts        INTEGER NOT NULL DEFAULT 0,
+    created_at      TEXT    NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ', 'now')),
+    next_attempt_at TEXT    NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ', 'now')),
+    last_error      TEXT
+);
+
+CREATE INDEX IF NOT EXISTS idx_event_queue_poll
+    ON event_queue (status, next_attempt_at);