refactor: store jt808 mileage as metric diff

docs/superpowers/specs/2026-06-23-gb32960-production-readiness-design.md

@@ -269,28 +269,20 @@ MYSQL_USERNAME
 MYSQL_PASSWORD
 ```
 
-`vehicle_daily_stat`
+Daily derived metrics are stored in the common metric table, not in a
+protocol-specific daily-mileage table.
 
 ```sql
-CREATE TABLE IF NOT EXISTS vehicle_daily_stat (
-  vin VARCHAR(32) NOT NULL,
+CREATE TABLE IF NOT EXISTS vehicle_stat_metric (
+  vin VARCHAR(64) NOT NULL,
   stat_date DATE NOT NULL,
-  platform_account VARCHAR(64),
-  total_mileage_km DECIMAL(14,3),
-  daily_mileage_km DECIMAL(14,3),
-  daily_power_kwh DECIMAL(14,3),
-  daily_hydrogen_kg DECIMAL(14,4),
-  daily_hydrogen_kg_per_100km DECIMAL(14,4),
-  first_event_time DATETIME(3),
-  last_event_time DATETIME(3),
-  first_total_mileage_km DECIMAL(14,3),
-  last_total_mileage_km DECIMAL(14,3),
-  last_soc_percent DECIMAL(8,3),
-  sample_count BIGINT NOT NULL DEFAULT 0,
-  late_sample_count BIGINT NOT NULL DEFAULT 0,
-  calculation_quality VARCHAR(32) NOT NULL,
-  updated_at DATETIME(3) NOT NULL,
-  PRIMARY KEY (vin, stat_date)
+  metric_key VARCHAR(64) NOT NULL,
+  metric_value DECIMAL(18,6) NULL,
+  metric_unit VARCHAR(16) NOT NULL,
+  calculation_method VARCHAR(64) NOT NULL,
+  created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  PRIMARY KEY (vin, stat_date, metric_key)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
 ```