feat: store pow performance statistics against statistics

db/db-sqlx-maria/migrations/20230627132800_mcaptcha_pow_analytics.sql

@@ -0,0 +1,13 @@
+CREATE TABLE IF NOT EXISTS mcaptcha_pow_analytics (
+	ID INT auto_increment,
+	PRIMARY KEY(ID),
+	config_id INTEGER NOT NULL,
+	time INTEGER NOT NULL,
+	difficulty_factor INTEGER NOT NULL,
+	worker_type VARCHAR(100) NOT NULL UNIQUE,
+	CONSTRAINT `fk_mcaptcha_config_id_pow_analytics`
+		FOREIGN KEY (config_id)
+		REFERENCES mcaptcha_config (config_id)
+		ON DELETE CASCADE
+		ON UPDATE CASCADE
+);