feat: add go daily metric writer

go/vehicle-gateway/internal/stats/schema.go

@@ -0,0 +1,20 @@
+package stats
+
+const DailyMetricTableSQL = `CREATE TABLE IF NOT EXISTS vehicle_daily_metric (
+  id BIGINT PRIMARY KEY AUTO_INCREMENT,
+  vin VARCHAR(32) NOT NULL,
+  stat_date DATE NOT NULL,
+  protocol VARCHAR(32) NOT NULL,
+  metric_key VARCHAR(64) NOT NULL,
+  metric_value DECIMAL(18,3) NOT NULL,
+  metric_unit VARCHAR(16) NOT NULL,
+  first_total_mileage_km DECIMAL(18,3) NULL,
+  latest_total_mileage_km DECIMAL(18,3) NULL,
+  sample_count BIGINT NOT NULL DEFAULT 0,
+  calculation_method VARCHAR(64) NOT NULL,
+  created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  updated_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
+  UNIQUE KEY uk_daily_metric (vin, stat_date, protocol, metric_key),
+  KEY idx_stat_date (stat_date),
+  KEY idx_protocol_metric (protocol, metric_key)
+)`