refactor: implement worker pool pattern, add persistent task queue, and decouple target loading logic

internal/db/db.go

@@ -55,7 +55,6 @@ func (db *DB) migrate() error {
 		location TEXT,
 		apply_url TEXT NOT NULL,
 		raw_content TEXT NOT NULL,
-		status TEXT NOT NULL DEFAULT 'PENDING',
 		crawled_at DATETIME DEFAULT CURRENT_TIMESTAMP
 	);
 
@@ -79,6 +78,18 @@ func (db *DB) migrate() error {
 		applied_at DATETIME,
 		updated_at DATETIME DEFAULT CURRENT_TIMESTAMP
 	);
+
+	CREATE TABLE IF NOT EXISTS tasks (
+		id INTEGER PRIMARY KEY AUTOINCREMENT,
+		task_type TEXT NOT NULL,
+		payload TEXT NOT NULL,
+		status TEXT NOT NULL DEFAULT 'PENDING',
+		error_message TEXT,
+		retry_count INTEGER DEFAULT 0,
+		next_retry_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+		created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+		updated_at DATETIME DEFAULT CURRENT_TIMESTAMP
+	);
 	`
 
 	_, err := db.Exec(schema)